Shylock in serial increasingly tight (9)

I believe the answer is:

slenderer

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'increasingly tight' is the definition.
I can't judge whether this definition defines the answer.

'shylock in serial' is the wordplay.
'shylock' becomes 'lender' (Shylock is a moneylender in The Merchant of Venice).
'in' is an insertion indicator.
'serial' becomes 'ser' (I can't justify this - if you can you should give a lot more credence to this answer).
'lender' put into 'ser' is 'SLENDERER'.

Can you help me to learn more?

(Another definition for slenderer that I've seen is " Increasingly slight".)

I've seen this clue in The Guardian.

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