Army corps is in the army, say, or part of the army (8)
I believe the answer is:
reserves
'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" .)