Crossword Genius

Must one pay flat fee? (6)

Ross

I believe the answer is:

lessee

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This could be a double definition.

'must one' is the first definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'pay flat fee?' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

Can you help me to learn more?

(Other definitions for lessee that I've seen before include "Tenant who holds a lease" , "One holding lease on property" , "One who rents" , "Leaseholder" , "Person who holds building on contract for specified time" .)

I've seen this clue in The Sun.
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