Mate with gun designed to extend (7)
I believe the answer is:
augment
'to extend' is the definition.
(thesaurus)
'mate with gun designed' is the wordplay.
'with' is a charade indicator (letters next to each other).
'designed' indicates an anagram (designed with the letters).
'mate' put after 'gun' is 'gunmate'.
'gunmate' anagrammed gives 'AUGMENT'.
(Other definitions for augment that I've seen before include "Add to, increase" , "Grow - intensify" , "Make larger" , "get longer" , "Expand" .)