Margaret Frew of 28 Washington Drive;Monessen;died Wednesday;Jan.29;1992;in Mon Valley Hospital;Carroll Township;following a lengthy illness.  Mrs.Frew was born in New Haven;Conn.;a daughter of the late William and Katherine Watson.  She had been a lifelong resident of Monessen.  Mrs. Frew was a member of St.Paul Lutheran Church;Monessen;and the Staunch & True Sunday School Class of the church.  She was also a member of Monessen Chapter No.205;Order of Eastern Star.  Surviving are her husband;Herbert F.Frew;one daughter;Mrs.Donald (Cheryl) DeVore;of Monongahela;a son and daughter-in-law;Dennis H.and Sadie Frew;of Hanover;three grandchildren;Benjamin and Lidsay DeVore and Dana Frew;and three step-grandchildren;Nanette Barberis;Daphanie Weaver and Stacy Martin.  Friends will be received Friday from 2 to 4 and 7 to 9 p.m.in the Rhome Funeral Home;Inc.;1209 Grand Blvd.;Monessen.  Services will be held Saturday at 1 p.m.in St.Paul Lutheran Church;Monessen;with Pastor Ronald Schtak officiating.  Burial will be in Belle Vernon Cemetery.  The Rhome Funeral Home;Inc.is in charge of arrangements.