Cowardly shout — ouch! (6)
I believe the answer is:
yellow
'cowardly' is the definition.
(yellow can informally mean cowardly)
'shout ouch' is the wordplay.
'shout' becomes 'yell' (yelling is a kind of shouting).
'ouch' becomes 'ow' (interjection synonym).
'yell'+'ow'='YELLOW'
(Other definitions for yellow that I've seen before include "Coldplay song; colour" , "likely to run" , "Primary colour" , "Daffodil colour" , "'Caution' colour" .)