Constant smell in inlet (5)
I believe the answer is:
creek
'inlet' is the definition.
(I've seen this before)
'constant smell' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'smell' becomes 'reek' (reek is a kind of smell).
'c'+'reek'='CREEK'
'in' acts as a link.
(Other definitions for creek that I've seen before include "Up the .....; in difficulties, and wet?" , "Rivulet" , "Small stream in the US" , "Narrow bay" , "Stream, brook" .)