Expert in Polish (4)
I believe the answer is:
buff
I believe this is a double definition.
'expert' is the first definition.
(buff can mean an expert or aficionado)
'polish' is the second definition.
(I know that buff is a more specific form of the action polish)
'in' is the link.
(Other definitions for buff that I've seen before include "Polish; expert, devotee" , "Bare skin - enthusiast" , "Dull yellow" , "Polish - one's bare skin" , "Finish" .)