Crossword Genius

In Idaho, a rare type of frost (4)

Ross

I believe the answer is:

hoar

'type of frost' is the definition.
(I've seen this before)

'in idaho a rare' is the wordplay.
'in' indicates the answer is hidden within the clue.
'HOAR' is hidden within 'idaho a rare'.

(Other definitions for hoar that I've seen before include "Grey with age; frost" , "Rime" , "Type of frost" , "A kind of white frost" , "Greyish-white with age or frost" .)

I've seen this clue in The Times.
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