Nothing to boast about in cooler (3,2,3)

I believe the answer is:

not so hot

I'm a little stuck... Click here to teach me more about this clue!

'to boast about in cooler' is the definition.
I can't tell whether this definition defines the answer.

'nothing' is the wordplay.
I cannot quite see how this works, but
'nothing' could be 'no' (similar in meaning) and 'no' is present in the answer.
This explanation may well be incorrect...

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