Hoar frost (4)
I believe the answer is:
rime
'hoar frost' 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 "Hoarfrost" , "dated poetry" , "Hoar frost" , "Kind of frost in mire" , "''The ... of the Ancient Mariner'' (Coleridge)" .)