Insect inside living naturally (4)
I believe the answer is:
gnat
'insect' is the definition.
(I know that gnat is a type of dipteran)
'inside living naturally' is the wordplay.
'inside' indicates a hidden word.
'GNAT' is hidden within 'living naturally'.
(Other definitions for gnat that I've seen before include "Small fly that bites" , "Insect like small mosquito" , "Small biter" , "Tiny thing" , "Small bloodsucker" .)