Challenge one member removing gun (6)
I believe the answer is:
impugn
'challenge' is the definition.
(impugning is a kind of challenging)
'one member removing gun' is the wordplay.
'one member' becomes 'imp' (I've seen this in other clues).
'removing' is an anagram indicator (moving the letters again).
'gun' is an anagram of 'ugn'.
'imp'+'ugn'='IMPUGN'
(Other definitions for impugn that I've seen before include "Blacken the name of" , "Try to discredit, express doubts about honesty" , "Attack verbally" , "Dispute the truth of" , "Attack as false, challenge" .)