Part of sign attracts small fly (4)
I believe the answer is:
gnat
'small fly' is the definition.
(I've seen this before)
'part of sign attracts' is the wordplay.
'part of' says the answer is hidden in the clue.
'GNAT' is hidden within 'sign attracts'.
(Other definitions for gnat that I've seen before include "a little bit of a nuisance" , "Insect like small mosquito" , "Insect often hovering over stagnant water" , "mozzie" , "small irritant" .)