Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
conscience [ˈkɔnʃəns] n со́весть f
he has a guilty/clear conscience у него́ со́весть нечиста́/чиста́
in all conscience по со́вести
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.