Cold coating (4)
I believe the answer is:
rime
'cold coating' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rime that I've seen before include "Ancient Mariner produced?" , "''The ... of the Ancient Mariner'' (Coleridge)" , "Frost left after freezing fog, or frozen dew" , "Ice" , "dated poetry" .)