Ruth A.Elder;84;of Kenric Manor;Donora;died Wednesday;July 12;1989;at 3:34 a.m.in Mon Valley Hospital.  Born Aug.2;1904;in Belle Vernon;she was the daughter of the late Charles and Stella Hammitt Mossburg.  Mrs.Elder had lived most of her adult life in the Belle Vernon area;and was a member of the Fayette City Christian Church.  In addition to her parents;she was preceded in death by her husband;Robert W.Elder;in 1979.  Surviving are one brother;Albert Mossburg;and one sister;Bertha Roberts;both of Belle Vernon.  Friends will be received Wednesday from 7 to 9 p.m.and Thursday from 2 to 4 and 7 to 9 p.m.;in the Ferguson Funeral Home Inc.;700 Broad Ave.;Belle Vernon;where services will be held Friday at 11 a.m.;with the Rev.Wayne Boring officiating.  Interment will follow in Belle Vernon Cemetery.  Arrangements are being handled by the Ferguson Funeral Home Inc.