Nasty smell associated with a snake (3)
I believe the answer is:
boa
'snake' is the definition.
(boa is a kind of snake)
'nasty smell associated with a' is the wordplay.
'nasty smell' becomes 'bo' (body odour).
'associated with' says to put letters next to each other.
'bo'+'a'='BOA'
(Other definitions for boa that I've seen before include "that's dangerous for South Americans" , "native American" , "One who puts the squeeze on" , "Large constricting snake" , "Large S American snake" .)