The House Social Security subcommittee has unanimously voted an important "grandmother clause" that would protect the Social Security spouses' benefit of women government workers and schoolteachers from the full effects of an offset due to go into effect in December 1982.

If approved by Congress, the language would provide for a gradual phase-in of the dollar-for-dollar offset formula (which already applies to most men) that is due to hit women who become eligible to retire from federal or public employment later next year.

Congress established the offset principle in law in 1977. But it applied only to men who get federal or public employment retirement benefits who are also eligible for a spouse's benefit, based on the Social Security earnings of their wives. At that time, Congress gave women until December 1982 to become eligible for retirement and still be able to collect the full amount of their spouses' Social Security benefits. The offset rule has no effect on an individual's earned benefits entitlement. It deals only with the so-called spouses benefit.

Last week the important subcommittee approved a plan that calls for a gradual phase-in of the offset formula which, under current law, is to be applied dollar-for-dollar to women beginning in late 1982.

Instead of reducing their spouses benefit by the full amount of their public pension or annuity, the subcommittee offset proposal would apply to only 20 percent of the public (or federal) pension for women who retire, or who become eligible to retire, between Dec. 1, 1982, and November 1983. The 20 percent offset would be permanent for them and not go up later on.

Women who retire or become eligible to retire from federal or public jobs after Dec. 1, 1982, and before November 1984 would be subject to a 40 percent offset formula that would not change for them. The offset would increase each year until the full, 100 percent, dollar-for-dollar offset takes effect for women retiring after November 1986.

Lobbyists, especially those representing powerful teachers organizations, have tried in vain to get Congress to exclude women completely from the offset formula for another five years. The subcommittee felt this gradual phase-in is as far as it can go. The unanimous vote on the lanugage means it has a good chance. Until it becomes law, women now faced with the choice of retiring before December 1982 or being hit with the full offset formula will be able to work longer and build bigger annuities while losing only a portion of their entitlement ot Social Security benefits their husbands earned.