Adjusted rent over a time within existing option (11)

I believe the answer is:

alternative

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'option' is the definition.
('alternative' can be a synonym of 'option')

'adjusted rent over a time within existing' is the wordplay.
'adjusted rent over' becomes 'tern' (I can't justify this - if you can you should believe this answer much more).
'time' becomes 't'.
'within' is an insertion indicator.
'existing' becomes 'alive' ('alive' can be similar in meaning to 'existing').
'tern'+'a'+'t'='ternat'
'ternat' placed into 'alive' is 'ALTERNATIVE'.

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(Other definitions for alternative that I've seen before include "Substitute" , "One possible option" , "(Of two things) mutually exclusive" , "One of two or more available possibilities" , "unavailable in Hobson's stable" .)

I've seen this clue in The Telegraph.

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