One-time undergarment? (7)
I believe the answer is:
singlet
'undergarment' is the definition.
(singlet is a kind of undergarment)
'one-time' is the wordplay.
'one' becomes 'single' ('single' can be a synonym of 'one').
'time' becomes 't'.
'single'+'t'='SINGLET'
(Other definitions for singlet that I've seen before include "Tingles in vest" , "Something to wear" , "Sleeveless shirt" , "garment worn by men" , "top gear" .)