Sham medicine put old boy back (7)
I believe the answer is:
placebo
'sham medicine' is the definition.
'placebo' can be an answer for 'medicine' (placebo is a kind of medicine). I am unsure of the 'sham' bit.
'put old boy back' is the wordplay.
'put' becomes 'place' (synonyms).
'old boy' becomes 'ob' (abbreviation).
'back' is a reversal indicator.
'ob' backwards is 'bo'.
'place'+'bo'='PLACEBO'
Can you help me to learn more?
(Other definitions for placebo that I've seen before include "Medical treatment whose effect is psychological" , "Medicine given for its psychological benefit alone" , "Medicine with no effect used in drug trials" , "Measure purely to calm or humour another" , "Medicament often given as a pacifier" .)