Insect observed coming back quietly (4)
I believe the answer is:
wasp
'insect' is the definition.
(I know that wasp is a type of insect)
'observed coming back quietly' is the wordplay.
'observed' becomes 'saw' (seen).
'coming back' shows that the letters should be reversed in order.
'quietly' becomes 'p' (p for piano and quiet).
'saw' written backwards gives 'was'.
'was'+'p'='WASP'
(Other definitions for wasp that I've seen before include "Being in the American elite" , "A hymenopteran" , "Influential American" , "Buzzer" , "Insect often irritating in summer" .)