Bug in a swarm (4)
I believe the answer is:
gnat
'bug in a swarm' 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 "Stinging insect" , "Sucker (if female)" , "little irritation" , "Two-winged fly" , "Small biting fly" .)