Muscle-building drug (7)
I believe the answer is:
steroid
'muscle-building drug' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for steroid that I've seen before include "Hormone perhaps" , "Type of hormone" , "Compound used as drug or performance enhancer" , "Body-building drug" , "Type of drug, maybe anabolic" .)