Quiet attendant who sells drugs (6)
I believe the answer is:
pusher
'quiet attendant who sells drugs' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'quiet attendant' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'attendant' becomes 'usher' (usher is a kind of attendant).
'p'+'usher'='PUSHER'
Can you help me to learn more?
(Other definitions for pusher that I've seen before include "A seller of drugs on the streets" , "Street seller of illegal drugs" , "Criminal who sells illegal drugs" , "See 20" .)