A quiet break interrupted by onset of loud cheers? (8)
I believe the answer is:
applause
'cheers?' is the definition.
(thesaurus)
'a quiet break interrupted by onset of loud' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'break' becomes 'pause' ('pause' can be a synonym of 'break'**).
'interrupted by' is an insertion indicator.
'onset of' indicates taking the first letters.
The first letter of 'loud' is 'l'.
'pause' enclosing 'l' is 'plause'.
'a'+'p'+'plause'='APPLAUSE'
(Other definitions for applause that I've seen before include "Burst of clapping" , "1a [BACALL] was in this musical" , "Clapping of hands" , "Audible indication of approval or praise" , "Appreciative round?" .)