In my opinion, it's rather mawkish (9)
I believe the answer is:
sentiment
'in' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I cannot understand how the rest of the clue works.
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" .)