Can return the egg (3)
I believe the answer is:
nit
'the egg' is the definition.
(nit is a kind of egg)
'can return' is the wordplay.
'can' becomes 'tin' (synonyms).
'return' says the letters should be written in reverse.
'tin' back-to-front is 'NIT'.
(Other definitions for nit that I've seen before include "Louse-egg" , "Louse's egg - berk" , "small egg" , "Young louse" , "Louse's egg; idiot" .)