Sharpen, as an appetite (4)
I believe the answer is:
whet
'sharpen as an appetite' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for whet that I've seen before include "Sharpen - stimulate" , "Make more acute" , "Make keen" , "Arouse" , "Sharpen by grinding" .)