In a compromise aimed at averting a veto, House-Senate conferees yesterday agreed to drop from the next fiscal year's public works money bill funds for the Clinch River breeder reactor, nine of 18 water projects, President Carter wants halted and 12 new water project construction starts.

Before taking their action, the conferees were reassurred yesterday morning that the president had told House-Speaker Thomas P. O'Neill Jr. (D-Mass.) that he would sign the legislation if it contained those adjustments.

The compromise was originally worked out between Carter and O'Neill, according to a member of the House leadership.

In completing work on the $10.4 billion measure, the House conferees also agreed to take back to the House for a seprate vote, a Senate-approved amendment under which Congress could override Carter if he eventually decides to go ahead with production of a new generation of neutron weapons.

Money to begin in building the neutron 8-inch shells and Lance missile warheads is contained in the Energy Research and Development Administration's portion of the bill.

The confeeres also dropped a House-passed provision that would have prohibited continued research and development work on a neutron version of the 155mm shell.

Although the President has said he wants both Clinch River and all 18 water projects on his "hit list" halted completely, the conferees made clear they considered their actions on the bill only temporary funding delays.

A report accompanying the conference committee's decisions will say that once the Clinch River reactor is authorized by legislation, a supplemental money bill with funds for it will be "immediately forthcoming."

Rather than killed by the conference, Sen. John C. Stennis (D-Miss.), chairman of the confrees said Clinch River was only "hemmed in for the time being."

As for the water projects, the conferees said funding for eight "hit list" projects originally approved by the House - it had 17 in its bills - but dropped by the Senate was only a delay until next year.

And the vote to eliminate new stars. Stennis added, did not mean "there will be no new starts in the future."

Enviornmental groups, which had given strong support to Carter's attempts to cut back on the congressional pork barrel system of approving water projects, reacted with anger to word of the public works bill compromise.

A spokesman for the Coalition for Water Project Review, made up primarily of national enviornmental and conservation groups, termed the compromise "a complete cave in" by the White House.

"We believe the President had betrayed his friends," the spokesman said, by not pushing Congress to drop all 18 of the "hit list" projects.

Disappointment was also voiced from the office of Rep. Butler C. Derrick (D.S.C.). Derrick had gone out on a limb joining Carter in opposition to the $267 million Richard Russell Dam which was on the "hit list" and in his district.

Russell project was one of the nine still funded by the compromise bill.

Derrick said yesterday he was assured on June 21 by Frank Moore, Carter's chief congressional liason, that the President would never sign the public works bill if it contained money for the Russell project.

That word, Derrick said, came "direct from the President" through Moore.It was delivered at a White House reception for Derrick and others who had supported Carter's position on water projects when the bill was on the House floor.

Derrick said Carter called him last Friday. At that time, according to Derrick, the President said he was thinking of signing the bill if the Clinch River money were eliminated.

"I reminded him of Moore's commitment," Derrick said yesterday. But "I don't recall he commented," the congressman added.

Derrick said a Carter decision to sign the bill with the Russell project in it "would not help my position" with constituents who want the dam built.

As of last nigh he still had not heard directly about the deal struck between the President and O'Neil.