Some foreign attachment to pesky insect (4)
I believe the answer is:
gnat
'insect' is the definition.
(I've seen this before)
'some foreign attachment to pesky' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'GNAT' is hidden in the letters of 'foreign attachment pesky'.
(Other definitions for gnat that I've seen before include "Stinging insect" , "annoying flier" , "Small bloodsucker" , "Woodland area" , "Insect like small mosquito" .)