To scream "That hurt!" is cowardly (6)
I believe the answer is:
yellow
'cowardly' is the definition.
('yellow' can be similar in meaning to 'cowardly')
'to scream that hurt' is the wordplay.
'to scream' becomes 'yell' ('yell' can be a synonym of 'scream'**).
'that hurt' becomes 'ow' (I've seen this in other clues).
'yell'+'ow'='YELLOW'
'is' is the link.
(Other definitions for yellow that I've seen before include "Primrose" , "Jaundiced - yolk" , "Shade of cowardice?" , "like old paper" , "Colour of the fever flag" .)