Suit part (4)
'suit part' is the definition.
('vest' is part of 'suit')
This is the entire clue.
(Other definitions for vest that I've seen before include "Upper undergarment, singlet" , "Undergarment to put in right of possession" , "Sleeveless undergarment" , "Place undergarment in fixed right of possession" , "Clothe in ecclesiastical garments" .)
'suit part' is the definition.
('waistcoat' is part of 'suit')
This is all the clue.
(Other definitions for waistcoat that I've seen before include "Part of a three-piece suit" , "Garment partly visible under a jacket" , "Something worn" , "Clothing" , "A "vest" in the US" .)