Upon retirement, received a one-piece garment (4)
I believe the answer is:
toga
'one-piece garment' is the definition.
(garment worn in ancient Rome)
'upon retirement received a' is the wordplay.
'upon retirement' shows that the letters should be reversed in order.
'received' becomes 'got' ('get' can be a synonym of 'receive').
'got' back-to-front is 'tog'.
'tog'+'a'='TOGA'
(Other definitions for toga that I've seen before include "Robe worn by Caesar" , "Graph's y value" , "Roman clothing" , "draped cloth" , "Roman gents' cloak" .)