Quietly increase applause (6)
I believe the answer is:
praise
'applause' is the definition.
(thesaurus)
'quietly increase' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'increase' becomes 'raise' (raising is a kind of increasing).
'p'+'raise'='PRAISE'
(Other definitions for praise that I've seen before include "Express approval and admiration" , "Commend or laud" , "Extol, glorify" , "Extol, laud" , "Kudos, pat on the back" .)