Intercede quietly to interrupt Stein (4,2)
I believe the answer is:
step in
'intercede' is the definition.
(thesaurus)
'quietly to interrupt stein' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'to interrupt' is an insertion indicator.
'p' going within 'stein' is 'STEP-IN'.
(Other definitions for step in that I've seen before include "Intervene dramatically, or at a senior level" , "Substitute" , "Interrupt" , "Shoe with no fastenings" , "Type of garment" .)