Source of colour initially absent in artificial silk (5)
I believe the answer is:
rayon
'artificial silk' is the definition.
(I've seen this before)
'source of colour initially absent' is the wordplay.
'source of colour' becomes 'crayon' (I've seen this before).
'initially absent' means to remove the first letter.
'crayon' with its first letter taken away is 'RAYON'.
'in' is the link.
(Other definitions for rayon that I've seen before include "Synthetic silk-like fabric" , "Silklike material" , "Semi-synthetic material" , "Viscose fibre" , "Viscose fabric" .)