Military force cracked when leader lost (4)

I believe the answer is:

army

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

I don't understand the remainder of the clue.

(Other definitions for army that I've seen before include "Military unit" , "Throng" , "service" , "Large host" , "Military land force" .)

I've seen this clue in The Times.

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