Frozen to the core? (4-7)

I believe the answer is:

cold-hearted

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'frozen to the core?' is the definition.
I can't judge whether this definition defines the answer.

'frozen to the core?' is the wordplay.
'frozen' becomes 'cold' (associated in meaning).
'to the core?' becomes 'hearted' (I can't justify this - if you can you should believe this answer much more).
'cold'+'hearted'='COLD-HEARTED'

Can you help me to learn more?

(Other definitions for cold-hearted that I've seen before include "Indifferent" , "Unkind" , "Unfeeling" .)

I've seen this clue in The Globe and Mail.

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