Yearn for new yellow (6)
I believe the answer is:
craven
'yellow' is the definition.
(both can mean cowardly)
'yearn for new' is the wordplay.
'yearn for' becomes 'crave' (I've seen this in another clue).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'crave'+'n'='CRAVEN'
(Other definitions for craven that I've seen before include "Lacking courage, cowardly" , "Faint-hearted" , "Contemptibly cowardly" , "Cowardly, spineless" , "Chicken" .)