Rank to arm head of nursing? (6)

I believe the answer is:

matron

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'head of nursing?' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for matron that I've seen before include "Old woman" , "Chief nurse" , "Senior nurse" , "Woman in charge of nurses" , "Nurses' manager" .)

I've seen this clue in the Sydney Morning Herald.

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