The colour of cowardice (6)
I believe the answer is:
yellow
'the colour of cowardice' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for yellow that I've seen before include "Cowardly; coloured" , "Lily-livered" , "Offensively sensational" , "like a canary" , "Colour of egg yolk and cowardice" .)