Gatekeeper holds back fencer's weapon (4)
I believe the answer is:
epee
'fencer's weapon' is the definition.
(I've seen this in another clue)
'gatekeeper holds back' is the wordplay.
'holds' indicates a hidden word.
'back' is a reversal indicator.
'eepe' is hidden in the letters of 'gatekeeper'.
'eepe' backwards is 'EPEE'.
(Other definitions for epee that I've seen before include "Another fencing sword" , "sports equipment" , "It may offer riposte" , "Fencing weapon" , "Duelling weapon" .)