Drill to polish up (3)
I believe the answer is:
bur
'drill' is the definition.
(small surgical drill)
'polish up' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'rub' backwards is 'BUR'.
'to' acts as a link.
(Other definitions for bur that I've seen before include "Prickly seed case" , "small drill" , "sticker" , "Seed-case" , "Clinging seed case" .)