Could be excessively so for school, weatherwise or attitude-wise. (4)

I believe the answer is:

cool

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'could' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for cool that I've seen before include "chic" , "Hip - not warm" , "Chilled" , "Lovely" , "Quite cold" .)

I've seen this clue in The Irish Times.

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