How octopus might be described in the military? (4)

I believe the answer is:

army

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

I cannot understand how the rest of the clue works.

(Other definitions for army that I've seen before include "See 7" , "Large host" , "Collective term for caterpillars" , "Military unit" , "Large military body" .)

I've seen this clue in The Times.

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