Unpleasant smell accompanies a swamp-dweller (3)
I believe the answer is:
boa
'swamp-dweller' is the definition.
'unpleasant smell accompanies a' is the wordplay.
'unpleasant smell' becomes 'bo' (informal abbreviation for body odour).
'accompanies' says to put letters next to each other.
'bo'+'a'='BOA'
(Other definitions for boa that I've seen before include "Large constrictor snake" , "strangler" , "Large snake provides a neck decoration" , "... cold-blooded creature" , "One who puts the squeeze on" .)