Source of sugar working for old cook (6)
I believe the answer is:
beeton
'old cook' is the definition.
(I've seen this before)
'source of sugar working' is the wordplay.
'source of sugar' becomes 'beet' (I've seen this before).
'working' becomes 'on'.
'beet'+'on'='BEETON'
'for' acts as a link.
(Other definitions for beeton that I've seen before include "Cookery expert" , "She helped us to cook" , "Mrs -, cookery writer" , "produced cookbook - with 19d" , "Famous cook" .)