Wolves cool in frozen mass (4-3)

I believe the answer is:

pack-ice

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'frozen mass' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'wolves cool' is the wordplay.
'wolves' becomes 'pack' (I can't justify this - if you can you should give a lot more credence to this answer).
'cool' becomes 'ice' (icing is a kind of cooling).
'pack'+'ice'='PACK-ICE'

'in' is the link.

Can you help me to learn more?

I've seen this clue in The Sun.

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