Waistcoat (4)
I believe the answer is:
vest
'waistcoat' is the definition.
(synonyms)
This is all the clue.
(Other definitions for vest that I've seen before include "Undergarment for upper body" , "Clothing" , "Clothe in ecclesiastical garments" , "Garment under a shirt" , "Undergarment to put in right of possession" .)