Sword keeper has knocked over (4)
I believe the answer is:
epee
'sword' is the definition.
(epee is a kind of sword)
'keeper has knocked over' is the wordplay.
'has' says the answer is hidden in the clue.
'knocked over' says the letters should be written in reverse.
'eepe' is hidden within 'keeper'.
'eepe' backwards is 'EPEE'.
(Other definitions for epee that I've seen before include "Fencer's sword" , "Fencing sword, heavier than a foil" , "there's a point to it" , "Fencing weapon" , "Foil for riposte" .)