Yellow head of cage bird (6)
I believe the answer is:
craven
'yellow' is the definition.
(thesaurus)
'head of cage bird' is the wordplay.
'head of' says to take the initial letters.
'bird' becomes 'raven' (raven is a kind of bird).
The first letter of 'cage' is 'c'.
'c'+'raven'='CRAVEN'
(Other definitions for craven that I've seen before include "Cowardly and fearful" , "Contemptibly cowardly" , "Chicken" , "Abjectly fearful" , "Lacking courage, cowardly" .)