Extremely ostentatious about unpleasant smells (6)
I believe the answer is:
odours
'smells' is the definition.
('odour' can be a synonym of 'smell')
'extremely ostentatious about unpleasant' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'about' is an insertion indicator.
'unpleasant' becomes 'dour' (similar in meaning).
'ostentatious' with its centre removed is 'os'.
'os' going around 'dour' is 'ODOURS'.
(Other definitions for odours that I've seen before include "Smells, usually unpleasant" , "Distinctive smells" , "Aromas, pungencies" , "Smells and fragrances" , "Smells, good or bad" .)