Burnt sienna (5)
I believe the answer is:
sepia
'burnt sienna' is the definition.
(I know that burnt sienna can be written as sepia)
This is the entire clue.
(Other definitions for sepia that I've seen before include "Rich reddish-brown colour" , "A greyish brown" , "Brown colour of old photo" , "Sort of ink" , "Colour of old photographs" .)