A small fly resembling a mosquito (4)
I believe the answer is:
gnat
I believe this clue is a double definition.
'a small fly' is the first definition.
(I've seen this before)
'a mosquito' is the second definition.
(gnat is a kind of mosquito)
'resembling' acts as a link.
(Other definitions for gnat that I've seen before include "annoying flier" , "Something with bite" , "Biting pest" , "Something that bites" , "mozzie" .)