A marine branch (3)

I believe the answer is:

arm

'branch' is the definition.
(thesaurus)

'a marine' is the wordplay.
'marine' becomes 'rm'.
'a'+'rm'='ARM'

(Other definitions for arm that I've seen before include "Test of strength with elbows on the table" , "Accoutre" , "Steel" , "Equip with weapons" , "Shoulder to fingertip" .)

I've seen this clue in the Evening Standard.

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