Challenge naughty child with fake gun (6)
I believe the answer is:
impugn
'challenge' is the definition.
(impugning is a kind of challenging)
'naughty child with fake gun' is the wordplay.
'naughty child' becomes 'imp' (informal term for a misbehaving child).
'with' is a charade indicator (letters next to each other).
'fake' is an anagram indicator (I've seen this in other clues).
'gun' with letters rearranged gives 'ugn'.
'imp'+'ugn'='IMPUGN'
(Other definitions for impugn that I've seen before include "Try to discredit, express doubts about honesty" , "Dispute the truth of" , "Attack as false, challenge" , "Challenge (e.g. honesty)" , "Dispute honesty of (someone's motive)" .)