Army corps is in the army, say, or part of the army (8)

I believe the answer is:

reserves

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'in the army say or part of the army' is the definition.
I can't judge whether this defines the answer.

'army corps' is the wordplay.
'army' becomes 'RE' (Royal Engineers regiment).
'corps' becomes 'serves' (I can't justify this - if you can you should give a lot more credence to this answer).
're'+'serves'='RESERVES'

'is' acts as a link.

Can you help me to learn more?

(Other definitions for reserves that I've seen before include "group of people for backup?" , "Books" , "Stockpiles" .)

I've seen this clue in The Times.

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