Keep quiet over substitute (8)

I believe the answer is:

preserve

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'keep' is the definition.
(I know that keep can be written as preserve)

'quiet over substitute' is the wordplay.
'quiet over' becomes 'p' (I can't justify this - if you can you should believe this answer much more).
'substitute' becomes 'reserve' (synonyms).
'p'+'reserve'='PRESERVE'

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(Other definitions for preserve that I've seen before include "put on the table in advance?" , "defend" , "Keep alive, maintain" , "Particular interest" , "Eg, jam" .)

I've seen this clue in The Telegraph.

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