Swoop (on) (6)
I believe the answer is:
pounce
'swoop on' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pounce that I've seen before include "Suddenly spring; fine powder" , "Spring (on prey like a cat)" , "Jump down on suddenly" , "(Of cat) sudden leap to seize prey" , "Jump or spring down as on prey" .)