Brings up half an army in reserves! (5)

I believe the answer is:

rears

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'brings up' is the definition.
('rear' can be a synonym of 'bring up')

'half an army in reserves' is the wordplay.
'half an' suggests halving the letters.
'army' becomes 'RE' (Royal Engineers regiment).
'in reserves' becomes 'ears' (I can't explain this - if you can you should give a lot more credence to this answer).
're' halved is 'r'.
'r'+'ears'='REARS'

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(Other definitions for rears that I've seen before include "Fosters, brings up" , "bucks" , "Sets up" , "Raises" , "Back ends" .)

I've seen this clue in the Evening Standard.

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