Swing at a mosquito (4)
I believe the answer is:
swat
'swing at a mosquito' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for swat that I've seen before include "Crush (eg, a fly)" , "Hit (insect) sharply" , "deliver blow" , "Hit (flies, etc)" , ".`Twas quite a blow" .)