Unmarried servant's last garment (7)
I believe the answer is:
singlet
'garment' is the definition.
(singlet is a kind of garment)
'unmarried servant's last' is the wordplay.
'unmarried' becomes 'single-' (similar in meaning).
'last' indicates one should take the final letters.
The last letter of 'servant' is 't'.
'single'+'t'='SINGLET'
(Other definitions for singlet that I've seen before include "Underwear" , "Tingles in vest" , "One unaccompanied" , "garment worn by men" , "Sleeveless shirt" .)