Shade in colour certainly retro (4)
I believe the answer is:
ecru
'shade' is the definition.
(colour)
'in colour certainly retro' is the wordplay.
'in' indicates the answer is hidden within the clue.
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'urce' is hidden in the letters of 'colour certainly'.
'urce' in reverse letter order is 'ECRU'.
(Other definitions for ecru that I've seen before include "one of the colours" , "Strange cure for a colour" , "Light fawn colour" , "Raw linen colour" , "No bright colour" .)