Allowed occupancy of some billets (3)

I believe the answer is:

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'allowed occupancy' is the definition.
'let' can be an answer for 'allowed' (to let is to allow or permit). I am not certain of the 'occupancy' bit.

'some billets' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'LET' can be found hidden inside 'billets'.

'of' is the link.

Can you help me to learn more?

(Other definitions for let that I've seen before include "Award (a contract)" , "Rent out (property)" , "Allow; rental period" , "Rent out: grant" , "Allow; leased" .)

I've seen this clue in the Evening Standard.

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