Jacket accompanier (4)
I believe the answer is:
vest
'jacket accompanier' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for vest that I've seen before include "Place undergarment in fixed right of possession" , "Undergarment for vets" , "Undershirt, singlet" , "Top" , "Upper undergarment, singlet" .)