A former clerk-typist at the Nigerian Embassy here pleaded guilty yesterday to giving an illegal payment of $1,500 to an Agriculture Department official in an alleged scheme to obtain a $57-million loan for rice shipments to Nigeria'

The woman, Thankam Mathews, a 43-year-old Indian national, pleaded guilty to giving the $1,500 as "good faith" money in an offer Dec. 6 to Lawrence T. McElvain, chief of the program operations branch of the Commercial export program of the Commodity Credit Corp.

Assistant U.S. Attorney David S' Krakoff told U.S. District Judge Aubrey E. Robinson Jr. that Mathews asked McElvain for help in financing shipment of 120.000 metric tons of rice from the United States to Nigeria. The prosecutor said that at first Mathews told McElvain that if he helped her with the loan "there will be $5,000 in it for you."

Krakoff said that eventually Mathews increased the offer to McElvain to $10,000 for each of 12 rice shipments of 10,000 metric tons, for a total of $120,000.

The prosecutor said that Mathews was arrested by the FBI at her apartment at 1234 Massachusetts Ave. NW shortly after she gave McElvain the $1,500. McElvain was wearing a tape recorder and a transmitter at the time, and FBI agents listened to the transaction outside the apartment in a truck, Krakoff said.