Cowardly scream "that hurt!" (6)
I believe the answer is:
yellow
'cowardly' is the definition.
(yellow can informally mean cowardly)
'scream that hurt' is the wordplay.
'scream' becomes 'yell' (yell can mean to scream or cry out).
'that hurt' becomes 'ow' (exclamation of pain).
'yell'+'ow'='YELLOW'
(Other definitions for yellow that I've seen before include "Daffodil colour" , "Cowardly - kind of fever" , "Shade of cowardice?" , "Cowardly; coloured" , "Chicken-hearted" .)