Finish, as a cake (5)
I believe the answer is:
frost
'finish as a cake' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for frost that I've seen before include "Ice crystals forming a white deposit" , "Robert ---, US poet" , "Chilly feeling" , "Frozen deposit" , "Coldness of manner" .)