Quietly he interrupts young relative (6)
I believe the answer is:
nephew
'relative' is the definition.
(nephew is a kind of relative)
'quietly he interrupts young' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'interrupts' indicates putting letters inside.
'young' becomes 'new' ('new' can be similar in meaning to 'young'**).
'p'+'he'='phe'
'phe' inserted inside 'new' is 'NEPHEW'.
(Other definitions for nephew that I've seen before include "Edward V to Richard III relation" , "Sibling's son" , "Has a duty (to do something)" , "Male relative" , "Sister's son" .)