Dirty smell set back prospect (7)
I believe the answer is:
obscene
'dirty' is the definition.
(similar in meaning)
'smell set back prospect' is the wordplay.
'smell' becomes 'B.O.' (abbreviation for Body Odour).
'set back' shows that the letters should be reversed in order.
'prospect' becomes 'scene' (synonyms).
'bo' in reverse letter order is 'ob'.
'ob'+'scene'='OBSCENE'
(Other definitions for obscene that I've seen before include "Shameless" , "Abhorrent or lewd" , "Offensively vulgar" , "offending sensibilities?" , "unsuitable for children?" .)