Military order some body parts (8,4)

I believe the answer is:

shoulder arms

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'military order' is the definition.
(I've seen this before)

'body parts' is the wordplay.
'body' becomes 'shoulder' (I can't justify this - if you can you should give a lot more credence to this answer).
'parts' becomes 'arms' (arm is a kind of part).
'shoulder'+'arms'='SHOULDER ARMS'

'some' acts as a link.

Can you help me to learn more?

(Another definition for shoulder arms that I've seen is " Military order".)

I've seen this clue in The Irish Times.

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