Dishearten gunner veteran to some extent (7)
I believe the answer is:
unnerve
'dishearten' is the definition.
(unnerve can mean to intimidate or dishearten)
'gunner veteran to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'UNNERVE' is hidden in the letters of 'gunner veteran'.
(Other definitions for unnerve that I've seen before include "To startle" , "Perturb" , "Dishearten - discourage" , "scare" , "Frighten" .)