Greyish-white with age or frost (4)
I believe the answer is:
hoar
'greyish-white with age or frost' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hoar that I've seen before include "Greyish-white with age or frost" , "A kind of white frost" , "Rime" , "Grey with age; frost" , "Type of frost" .)