Take on when birds rise (6)
I believe the answer is:
assume
'take on' is the definition.
(I know that take on can be written as assume)
'when birds rise' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'birds' becomes 'emus' (emu is a kind of bird).
'rise' is a reversal indicator.
'emus' back-to-front is 'sume'.
'as'+'sume'='ASSUME'
(Other definitions for assume that I've seen before include "Be arrogant" , "Accept as true without proof" , "Take as true or take for granted" , "Tale of the gods" , "Take to be the case" .)