Expert's device for adding polish (4)
I believe the answer is:
I believe this clue is a double definition.
'expert's' is the first definition.
(buff can mean an expert or aficionado)
'device for adding polish' is the second definition.
(a pad used to polish or buff something)
(Other definitions for buff that I've seen before include "Pale yellow" , "light shade" , "Adept; yellowish-brown" , "Yellowish-beige; expert" , "Dull yellow" .)