Guess when birds returned (6)
I believe the answer is:
assume
'guess' is the definition.
(I've seen this before)
'when birds returned' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'birds' becomes 'emus' (emu is a kind of bird).
'returned' shows that the letters should be reversed in order.
'emus' reversed gives 'sume'.
'as'+'sume'='ASSUME'
(Other definitions for assume that I've seen before include "Tale of the gods" , "Dark brown fur" , "Take for granted or agreed" , "Take as true or take for granted" , "Take on, affect" .)