Putting one over on the syndicate (5)

I believe the answer is:

union

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'syndicate' is the definition.
(I know that union is a type of syndicate)

'putting one over on' is the wordplay.
'putting one over' becomes 'uni' (I can't explain this - if you can you should give a lot more credence to this answer).
'uni'+'on'='UNION'

'the' is the link.

Can you help me to learn more?

(Other definitions for union that I've seen before include "Coupling, merger" , "political alliance" , "Workers' federation" , "Trade organisation to represent workers" , "America" .)

I've seen this clue in the Evening Standard.

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