In my opinion, it's rather mawkish (9)

I believe the answer is:

sentiment

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'in' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for sentiment that I've seen before include "Tender or romantic emotion" , "Mawkishness" , "Emotional feeling" , "Emotional thought" , "View, opinion, feeling" .)

I've seen this clue in the Evening Standard.

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