Transform mostly silent gallery (6)
I believe the answer is:
mutate
'transform' is the definition.
(thesaurus)
'mostly silent gallery' is the wordplay.
'mostly' means to remove the last letter.
'silent' becomes 'mum' ('mum' can be a synonym of 'silent').
'gallery' becomes 'tate' (I've seen this before).
'mum' with its last letter taken off is 'mu'.
'mu'+'tate'='MUTATE'
(Other definitions for mutate that I've seen before include "(In genetics) change" , "Alter genetically" , "Change genetic structure" , "Change in basic nature" , "effect major change" .)