Ineffective medicine's rank smell (7)
I believe the answer is:
placebo
'ineffective medicine's' is the definition.
(I've seen this before)
'rank smell' is the wordplay.
'rank' becomes 'place' (synonyms).
'smell' becomes 'B.O.' (abbreviation for Body Odour).
'place'+'bo'='PLACEBO'
(Other definitions for placebo that I've seen before include "Something used to calm or humour a person" , "Inactive control used in drug testing" , "Something done to please" , "control measure?" , "Inactive treatment" .)