Bad smell from sulphur and fish (6)
I believe the answer is:
stench
'bad smell' is the definition.
(I've seen this before)
'sulphur and fish' is the wordplay.
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'and' says to put letters next to each other.
'fish' becomes 'tench' (tench is a kind of fish).
's'+'tench'='STENCH'
'from' acts as a link.
(Other definitions for stench that I've seen before include "A really bad smell" , "Unpleasant odour" , "smell, to put it mildly" , "Foul odour" , "Strong offensive smell" .)