One letter brings underwear (7)
I believe the answer is:
singlet
'underwear' is the definition.
(I know that singlet is a type of undershirt)
'one letter' is the wordplay.
'one' becomes 'single' (synonyms).
'letter' becomes 't' ().
'single'+'t'='SINGLET'
'brings' acts as a link.
(Other definitions for singlet that I've seen before include "underclothing" , "One unaccompanied" , "garment worn by men" , "Sleeveless shirt" , "Sleeveless vest" .)