Irksome insect (4)
I believe the answer is:
gnat
'irksome insect' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gnat that I've seen before include "Small biting flying insect" , "Small biting fly" , "Biting pest" , "Sucker (if female)" , "Two-winged fly" .)