Not old enough to destroy deer with a gun (8)
I believe the answer is:
underage
'not old enough' is the definition.
'destroy deer with a gun' is the wordplay.
'destroy' is an anagram indicator (I've seen this in other clues).
'with' means one lot of letters go next to another.
'a'+'gun'='agun'
'deer' put after 'agun' is 'agundeer'.
'agundeer' with letters rearranged gives 'UNDERAGE'.
'to' acts as a link.
(Other definitions for underage that I've seen before include "Not yet eligible to vote - too young" , "Need a rug (anag.)" , "Too tender" , "Not legally old enough" , "Immature" .)