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Set of rooms beside theatre, originally housing soldiers (9)

Ross

I believe the answer is:

apartment

I'm a little stuck... Click here to teach me more about this clue!

'set of rooms' is the definition.
(I've seen this before)

'beside theatre originally housing soldiers' is the wordplay.
I cannot really understand how this works, but
'theatre' could be 'nt' (National Theatre) and 'nt' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for apartment that I've seen before include "Room ? flat" , "Suite of rooms to live in" , "Set of rooms or American flat" , "property" , "Penthouse" .)

I've seen this clue in The Telegraph.
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