Turn yellow bird away (7,3)
I believe the answer is:
chicken out
'turn yellow' is the definition.
(I've seen this in another clue)
'bird away' is the wordplay.
'bird' becomes 'chicken' (chicken is a kind of bird).
'away' becomes 'out' (similar in meaning).
'chicken'+'out'='CHICKEN OUT'
(Other definitions for chicken out that I've seen before include "Be a wimp" , "bottle it" , "When yellow, withdraw" , "Back down" , "turn yellow" .)