Stream good alongside bad (4)
I believe the answer is:
gill
'stream' is the definition.
(thesaurus)
'good alongside bad' is the wordplay.
'good' becomes 'g' (abbreviation).
'alongside' means one lot of letters go next to another.
'bad' becomes 'ill' (associated in meaning).
'g'+'ill'='GILL'
(Other definitions for gill that I've seen before include "Fish's breathing apparatus" , "ravine" , "Organ for breathing under water" , "Respiratory organ of fish" , "A little drink" .)