It's cold in ruined forts

I believe the answer is:

frost

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'it's cold' is the definition.
I can't tell whether this defines the answer.

'ruined forts' is the wordplay.
'ruined' indicates an anagram.
'forts' is an anagram of 'FROST'.

'in' acts as a link.

Can you help me to learn more?

(Other definitions for frost that I've seen before include "Frozen deposit" , "Icy condensation" , "RD Wingfield's detective" , "Icy deposit" , "Frozen dew" .)

I've seen this clue in the Evening Standard.

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