Sharpen (with a whetstone) (4)
I believe the answer is:
hone
'sharpen with a whetstone' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hone that I've seen before include "Strop" , "Sharpen tool on stone" , "Polish" , "Sharpen (a blade)" , "Smooth stone" .)