Are reps and a disappointment (4)

I believe the answer is:

sell

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'disappointment' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see an association between them that I can't see?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for sell that I've seen before include "Flog" , "Exchange for cash" , "Trade" , "disappointment?" , "Decorative edging" .)

I've seen this clue in the Evening Standard.

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