Clue: It will purge 1 down of muscly stereotypes (7)
Answer: CLYSTER
Explanation: Definition: It will purge 1 down

Hidden in [*of*] "mus**CLY STER**eotypes"
 
"Clyster" is an archaic term for an enema, and the clue should have indicated that it is an archaic word. For example, "It will" could have been changed to "It used to…"