Round of applause's volume in social gathering reduced (5)
I believe the answer is:
salvo
'round of applause's' is the definition.
(I've seen this before)
'volume in social gathering reduced' is the wordplay.
'volume' becomes 'v' (abbreviation for volume in book titles).
'in' indicates putting letters inside.
'social gathering' becomes 'salon' (salon can mean a get-together).
'reduced' means to remove the last letter.
'salon' with its final letter removed is 'salo'.
'v' inserted inside 'salo' is 'SALVO'.
(Other definitions for salvo that I've seen before include "Broadside" , "Ceremonial round of gunfire" , "Fiery outburst" , "Sudden round of applause" , "Succession of shots" .)