Referee briefly popped out for a smoke (6)
I believe the answer is:
reefer
'for a smoke' is the definition.
(reefer is a kind of smoke)
'referee briefly popped' is the wordplay.
'briefly' means to remove the last letter.
'popped' indicates anagramming the letters (I've seen 'popping' mean this (the original order pops or explodes)).
'referee' with its final letter taken away is 'refere'.
'refere' anagrammed gives 'REEFER'.
'out' acts as a link.
(Other definitions for reefer that I've seen before include "Easy" , "Something designed for smoking" , "Joint; spliff" , "roll of hemp" , "A sailor's jacket or marijuana cigarette" .)