Basin and toilet with a nasty smell (6)
I believe the answer is:
lavabo
'basin' is the definition.
(lavabo is a kind of basin)
'toilet with a nasty smell' is the wordplay.
'toilet' becomes 'lav' (synonyms).
'with' says to put letters next to each other.
'nasty smell' becomes 'B.O.' (abbreviation for body odour).
'lav'+'a'+'bo'='LAVABO'
'and' acts as a link.
(Other definitions for lavabo that I've seen before include "Catholic washing" , "bowl" , "Toot" , "Mass cleansing" , "Washing trough" .)