Small insect somewhat irritating, naturally (4)
I believe the answer is:
gnat
'small insect' is the definition.
(I've seen this in another clue)
'somewhat irritating naturally' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'GNAT' is hidden within 'irritating naturally'.
(Other definitions for gnat that I've seen before include "Sucker (if female)" , "annoying flier" , "little irritation" , "Insect often hovering over stagnant water" , "a little bit of a nuisance" .)