Frost in Crimea (4)
I believe the answer is:
rime
'frost' is the definition.
(rime is a type of frost)
'in crimea' is the wordplay.
'in' indicates a hidden word.
'RIME' is hidden within 'crimea'.
(Other definitions for rime that I've seen before include "Hoarfrost" , "Hoar frost" , "Kind of frost in mire" , "''The ... of the Ancient Mariner'' (Coleridge)" , "Ice" .)