Form leaves clothes at school for after-school activity (3)

I believe the answer is:

uni

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'form' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for uni that I've seen before include "Seat of learning" , "academic establishment" , "Where you can study" , "Queen's, Belfast, for example" , "Number of colleges" .)

I've seen this clue in The Independent.

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