Sound of insect or fly (4)
I believe the answer is:
flee
'fly' is the definition.
(synonyms)
'sound of insect' is the wordplay.
'sound of' indicates a 'sounds like' (homophone) clue.
'insect' becomes 'flea' (flea is a kind of insect).
'flea' is a homophone of 'FLEE'.
'or' acts as a link.
(Other definitions for flee that I've seen before include "Run (from danger)" , "Leave" , "rush" , "head for the hills" , "Cut and run" .)