Strong unpleasant smell (6)
'strong unpleasant smell' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for stench that I've seen before include "A really bad smell" , "Appalling smell" , "Awful smell" , "Miasma" , "Unpleasant odour" .)
(Other definitions for stink that I've seen before include "Bad smell" , "Unpleasant smell" , "Terrible smell" , "Hum" , "Pong" .)