Frosty coating (4)
I believe the answer is:
rime
'frosty coating' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rime that I've seen before include "Ancient Mariner produced?" , "Ice" , "Kind of frost in mire" , "Hoarfrost" , "''The ... of the Ancient Mariner'' (Coleridge)" .)