Air gun set new mark (9)
I believe the answer is:
signature
'mark' is the definition.
(I have seen 'Distinctive mark ' mean 'signature' so perhaps 'mark' could also mean 'signature')
'air gun set new' is the wordplay.
'air gun' becomes 'airgun' (synonyms).
'new' indicates an anagram.
'airgun'+'set'='airgunset'
'airgunset' anagrammed gives 'SIGNATURE'.
(Other definitions for signature that I've seen before include "distinctive style" , "Squiggle perhaps" , "proof of identity" , "Handwritten name" , "Personal mark" .)