Challenge one politician brandishing gun (6)
I believe the answer is:
impugn
'challenge' is the definition.
(impugning is a kind of challenging)
'one politician brandishing gun' is the wordplay.
'one politician' becomes 'imp' (I've seen this in other clues).
'brandishing' indicates anagramming the letters (I've seen 'brandished' mean this (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 "Blacken the name of" , "Dispute the truth of" , "Call into question or attack by words" , "Challenge (e.g. honesty)" , "Express doubts about truth or honesty of, say motives" .)