Withdraw control over very quiet youngster (6)
I believe the answer is:
nipper
'youngster' is the definition.
(I know that youngster can be written as nipper)
'withdraw control over very quiet' is the wordplay.
'withdraw' is a reversal indicator.
'control' becomes 'rein' (synonyms).
'over' means one lot of letters goes inside another.
'very quiet' becomes 'pp' (musical abbreviation for pianissimo).
'rein' enclosing 'pp' is 'reppin'.
'reppin' reversed gives 'NIPPER'.
(Other definitions for nipper that I've seen before include "Little chap" , "A pincer or a young boy" , "Slang boy or lobster claw" , "Crab - young shaver?" , "Child (slang); one that 19s" .)