Free the Queen or Knight? (5)

I believe the answer is:

rider

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'knight?' is the definition.
(thesaurus)

'free the queen' is the wordplay.
'free' becomes 'ride' (I can't explain this - if you can you should give a lot more credence to this answer).
'the queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'ride'+'r'='RIDER'

'or' acts as a link.

Can you help me to learn more?

(Other definitions for rider that I've seen before include "Passenger" , "Comment from jury" , "Supplementary clause" , "Gold coin" , "Proviso in a legal document" .)

I've seen this clue in The Independent.

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