Type of frost (4)
I believe the answer is:
hoar
'type of frost' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hoar that I've seen before include "Greyish-white with age or frost" , "Type of frost" , "Rime" , "Here are ice crystals" , "A kind of white frost" .)