Mostly remain in disgrace (5)
I believe the answer is:
stain
'disgrace' is the definition.
(thesaurus)
'mostly remain in' is the wordplay.
'mostly' means to remove the last letter.
'remain' becomes 'stay' (staying is a kind of remaining).
'stay' with its final letter removed is 'sta'.
'sta'+'in'='STAIN'
(Other definitions for stain that I've seen before include "Colour, dye" , "Tarnish" , "Stigma" , "Blot" , "Discolouring mark" .)