Boxer receiving left punch (4)
I believe the answer is:
plug
'punch' is the definition.
(synonyms)
'boxer receiving left' is the wordplay.
'boxer' becomes 'pug' (short for pugilist - term for boxer).
'receiving' indicates putting letters inside.
'left' becomes 'l' (common abbreviation).
'pug' going around 'l' is 'PLUG'.
(Other definitions for plug that I've seen before include "Device put into socket" , "Good word" , "Publicise -- stop" , "Wad of tobacco and electrical accessory" , "Promote" .)