Slip on waistcoat, not current undergarment (7)
I believe the answer is:
singlet
'undergarment' is the definition.
(I know that singlet is a type of undergarment)
'slip on waistcoat not current' is the wordplay.
'slip' becomes 'sin' (both can mean a mistake or failing).
'on' is a charade indicator (letters next to each other).
'waistcoat' becomes 'gilet' (a gilet is a sleeveless jacket).
'not' suggests deleting specific letters.
'current' becomes 'I' (physics symbol).
'gilet' with 'i' removed is 'glet'.
'sin'+'glet'='SINGLET'
(Other definitions for singlet that I've seen before include "Something to wear" , "One unaccompanied" , "Sleeveless shirt" , "garment worn by men" , "Sleeveless vest" .)