Swarming round branch (3)

I believe the answer is:

arm

I'm a little stuck... Click here to teach me more about this clue!

'branch' is the definition.
(synonyms)

'swarming round' is the wordplay.
'ARM' is hidden within the letters.
But, I'm unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for arm that I've seen before include "What soldiers do" , "Maybe rifle" , "Provide with weapons" , "Upper limb" , "Accoutre" .)

I've seen this clue in The Mirror.

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