Swoop, fall upon (6)
I believe the answer is:
pounce
I believe this clue is a double definition.
'swoop' is the first definition.
(I know that swoop can be written as pounce)
'fall upon' is the second definition.
(thesaurus)
(Other definitions for pounce that I've seen before include "Jump down on suddenly" , "Spring or swoop down on to attack" , "Strike" , "Spring (on prey like a cat)" , "Jump swiftly on, as cat might" .)