They rank the performances of association used by mathematicians (6,6)

I believe the answer is:

league tables

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'they rank the performances' is the definition.
The definition suggests an adverb but the answer is not.

'association used by mathematicians' is the wordplay.
'association' becomes 'league' (I've seen this before).
'used by mathematicians' becomes 'tables' (I am not sure about this - if you are sure you should believe this answer much more).
'league'+'tables'='LEAGUE TABLES'

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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