Garment for unmarried head teacher (7)

I believe the answer is:

singlet

'garment' is the definition.
(singlet is a kind of garment)

'unmarried head teacher' is the wordplay.
'unmarried' becomes 'single-' (similar in meaning).
'head' says to take the initial letters.
The initial letter of 'teacher' is 't'.
'single'+'t'='SINGLET'

'for' is the link.

(Other definitions for singlet that I've seen before include "underclothing" , "Sleeveless shirt" , "One unaccompanied" , "Tingles in vest" , "Underwear" .)

I've seen this clue in The Globe and Mail.

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