Undergarment, singlet (4)
I believe the answer is:
vest
This clue is a double definition.
'undergarment' is the first definition.
(vest is a kind of undergarment)
'singlet' is the second definition.
(synonyms)
(Other definitions for vest that I've seen before include "Sleeveless garment" , "Undergarment to put in right of possession" , "American version of waistcoat" , "Sleeveless undergarment" , "Thesaurus compiler" .)