Doctor protects drug, making injection for certain people only (3,6)
I believe the answer is:
top secret
'injection for certain people only' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'doctor protects drug' is the wordplay.
'doctor' indicates an anagram.
'drug' becomes 'e' (e is another name for ecstasy).
'protects'+'e'='protectse'
'protectse' anagrammed gives 'TOP-SECRET'.
'making' acts as a link.
Can you help me to learn more?
(Other definitions for top secret that I've seen before include "the knack of sleeping well?" , "Highest classified (information)" , "As Bond mission" , "Erect post (anag)" , "Very hush-hush" .)