Constant foul smell in bay (5)
I believe the answer is:
creek
'bay' is the definition.
(thesaurus)
'constant foul smell' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'foul smell' becomes 'reek' (I've seen this before).
'c'+'reek'='CREEK'
'in' is the link.
(Other definitions for creek that I've seen before include "Small water course" , "Narrow inlet from the sea" , "Up the .....; in difficulties, and wet?" , "Small inlet(no place to be up!)" , "Natural stream" .)