Retaliate for some craven gentleman (6)
I believe the answer is:
avenge
'retaliate for' is the definition.
(I've seen this before)
'some craven gentleman' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'AVENGE' can be found hidden inside 'craven gentleman'.
(Other definitions for avenge that I've seen before include "Get back at for (a wrong)" , "Get one's own back for" , "Gain retribution" , "Dish out a dose of own medicine" , "Even the score" .)