Dummy's rank smell (7)
I believe the answer is:
placebo
'dummy's' is the definition.
(ineffective or dummy version of a drug)
'rank smell' is the wordplay.
'rank' becomes 'place' (synonyms).
'smell' becomes 'B.O.' (informal abbreviation for body odour).
'place'+'bo'='PLACEBO'
(Other definitions for placebo that I've seen before include "Inert drug" , "Ineffective substance given to humour patient" , "Innocuous or inert medication" , "patient gratified to receive such" , "Innocuous medication; palliative" .)