Carefully protect Peter mixing a drug (9)
I believe the answer is:
safeguard
'carefully protect' is the definition.
'peter mixing a drug' is the wordplay.
'peter' becomes 'safe' (slang term).
'mixing' indicates anagramming the letters.
'a'+'drug'='adrug'
'adrug' with letters rearranged gives 'guard'.
'safe'+'guard'='SAFEGUARD'
(Other definitions for safeguard that I've seen before include "Preserve" , "Protect" , "Protective measure" .)