Attack a troublemaker with a gun perhaps (6)
I believe the answer is:
impugn
'attack' is the definition.
(I've seen this before)
'a troublemaker with a gun perhaps' is the wordplay.
'a troublemaker' becomes 'imp' (I've seen this in another clue).
'with' says to put letters next to each other.
'perhaps' indicates anagramming the letters.
'gun' is an anagram of 'ugn'.
'imp'+'ugn'='IMPUGN'
(Other definitions for impugn that I've seen before include "Try to discredit, express doubts about honesty" , "Express doubts about truth or honesty of, say motives" , "Blacken the name of" , "Nip mug badly and call in question" , "Attack as false, challenge" .)