Break matron's little window (7)

I believe the answer is:

transom

'little window' is the definition.
(transom window is a type of small window)

'break matron's' is the wordplay.
'break' indicates anagramming the letters.
'matrons' is an anagram of 'TRANSOM'.

(Other definitions for transom that I've seen before include "Crossbar over door" , "timber for doorframe" , "bolstering member" , "Bar above window or door" , "Strengthening crossbar" .)

I've seen this clue in The Guardian.

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