Aggressive US cop with a gun, I suspect (10)
I believe the answer is:
pugnacious
'aggressive' is the definition.
('pugnacious' can be similar in meaning to 'aggressive')
'us cop with a gun i suspect' is the wordplay.
'with' is a charade indicator (letters next to each other).
'suspect' indicates an anagram.
'us'+'cop'+'a'+'gun'+'i'='uscopaguni'
'uscopaguni' is an anagram of 'PUGNACIOUS'.
(Other definitions for pugnacious that I've seen before include "Quarrelsome" , "Combative" , "Aggressive" .)