Improve, fine-tune (6)
I believe the answer is:
refine
This clue is a double definition.
'improve' is the first definition.
(I know that refine is a more specific form of improve)
'fine-tune' is the second definition.
(I know that fine-tune can be written as refine)
(Other definitions for refine that I've seen before include "Remove impurities or make cultured" , "Remove impurities from (eg, metals)" , "Filter or process as with oil" , "Remove impurities as from oil" , "Get rid of impurities" .)