Develop part of a revolver (6)
I believe the answer is:
evolve
'develop' is the definition.
(I know that evolve has the is a more specific form of the action develop)
'part of a revolver' is the wordplay.
'part of' indicates a hidden word.
'EVOLVE' can be found hidden inside 'revolver'.
(Other definitions for evolve that I've seen before include "Gradually develop as Darwin predicted" , "Develop by natural selection" , "Unravel" , "Slowly change" , "Cartoon sailor" .)