Imagine an order to join the army (4,2)

I believe the answer is:

call up

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'join the army' is the definition.
'call up' can be an answer for 'join' (thesaurus). I'm not sure about the 'the army' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for call up that I've seen before include "enlist" , "Order to report for military duty" , "Evoke memories" , "Summon; evoke" , "Ring" .)

I've seen this clue in The Globe and Mail.

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