Develop gun with no edges (6)
I believe the answer is:
evolve
'develop' is the definition.
(evolving is a kind of developing)
'gun with no edges' is the wordplay.
'gun' becomes 'revolver' (revolver is a kind of gun).
'with no edges' indicates the central letters (edge letters are removed).
The centre of 'revolver' is 'EVOLVE'.
(Other definitions for evolve that I've seen before include "Gradually acquire new characteristics" , "Unravel" , "Unfold" , "Slowly change" , "Develop gradually over a long time" .)