Reduced force includes soldiers on board of the highest quality (11)

I believe the answer is:

primariness

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'reduced force' is the definition.
The definition suggests a singular noun which matches the answer.

'soldiers on board of the highest quality' is the wordplay.
'soldiers' becomes 'marines' (I've seen this before).
'on' means one lot of letters goes inside another.
'board of the highest quality' becomes 'pris' (I can't explain this - if you can you should give a lot more credence to this answer).
'marines' going inside 'pris' is 'PRIMARINESS'.

'includes' acts as a link.

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I've seen this clue in The Times.

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