Smack, as a mosquito (4)
I believe the answer is:
swat
'smack as a mosquito' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for swat that I've seen before include "Hit smartly" , "Hit with a sharp blow" , "deliver blow" , "Hit a wasp or fly" , "District of Pakistan; sharp blow" .)