Yellow bird at front of cage (6)
I believe the answer is:
craven
'yellow' is the definition.
(both can mean cowardly)
'bird at front of cage' is the wordplay.
'bird' becomes 'raven' (raven is a kind of bird).
'at' means one lot of letters go next to another (I've seen this in other clues).
'front of cage' becomes 'c' (I can't justify this - if you can you should give a lot more credence to this answer).
'raven' put after 'c' is 'CRAVEN'.
(Other definitions for craven that I've seen before include "Faint-hearted" , "Abjectly fearful" , "Cowardly, spineless" , "Chicken-hearted" , "Contemptibly cowardly" .)