Dummy pill (7)
I believe the answer is:
placebo
'dummy pill' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for placebo that I've seen before include "Innocuous medication; palliative" , "Innocuous or inert medication" , "Measure purely to calm or humour another" , "ineffective medicine" , "Medicine working by psychology only" .)