Fly over low zone (4)
I believe the answer is:
zoom
'fly' is the definition.
(I've seen this before)
'over low zone' is the wordplay.
'over' says the letters should be written in reverse.
'low' becomes 'moo' (both can mean to make a cow noise).
'zone' becomes 'z'.
'moo'+'z'='mooz'
'mooz' reversed gives 'ZOOM'.
(Other definitions for zoom that I've seen before include "Belt along" , "Change focus smoothly" , "Whizz - camera lens" , "go very fast" , "Rush around" .)