Believed unpleasant smell that's disgusting lost at last (6)
I believe the answer is:
bought
'believed' is the definition.
('buy' can be a synonym of 'believe')
'unpleasant smell that's disgusting lost at last' is the wordplay.
'unpleasant smell' becomes 'BO' (informal abbreviation for body odour).
'that's disgusting' becomes 'ugh' (expression of disgust).
'at last' indicates one should take the final letters.
The last letter of 'lost' is 't'.
'bo'+'ugh'+'t'='BOUGHT'
(Other definitions for bought that I've seen before include "Obtained" , "paid for" , "Puchased" , "Purchased" .)