Cooperation about a rent negotiated between here and Mars, say (14)

I believe the answer is:

interplanetary

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'cooperation about a rent negotiated' is the definition.
I can't tell whether this definition defines the answer.

'between here and mars say' is the wordplay.
'between' becomes 'inter'.
'here' becomes 'ary' (I can't justify this - if you can you should believe this answer much more).
'and' says to put letters next to each other.
'mars say' becomes 'planet' (Mars is an example).
'ary' put after 'planet' is 'planetary'.
'inter'+'planetary'='INTERPLANETARY'

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I've seen this clue in The Daily Express.

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