Jumpers turned up by Leicester's number one fans (9)

I believe the answer is:

pullovers

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'jumpers' is the definition.
(I know this)

'turned up by leicester's number one fans' is the wordplay.
'turned up by leicester's number' becomes 'pul' (I can't explain this - if you can you should give a lot more credence to this answer).
'one fans' becomes 'lovers' ('lover' can be a synonym of 'fan').
'pul'+'lovers'='PULLOVERS'

Can you help me to learn more?

I've seen this clue in The Guardian.

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