Nick is more self-possessed (6)

I believe the answer is:

cooler

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'nick' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for cooler that I've seen before include "less enthusiastic" , "Nick" , "It's not so hot" , "Prison - less warm" , "more sophisticated" .)

I've seen this clue in the Evening Standard.

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