Resolve of partner no longer restrained (7)

I believe the answer is:

explain

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'resolve' is the definition.
(I've seen this before)

'partner no longer restrained' is the wordplay.
'partner no longer' becomes 'ex' (ex-partner).
'restrained' becomes 'plain' (I can't explain this - if you can you should believe this answer much more).
'ex'+'plain'='EXPLAIN'

'of' is the link.

Can you help me to learn more?

(Other definitions for explain that I've seen before include "Show the meaning of, make clear" , "Give reason for" , "Defend" , "Interpret" , "offer clarification" .)

I've seen this clue in The Sunday Times and The Times.

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