Cry on hearing social gathering (4)
I believe the answer is:
bawl
'cry' is the definition.
(I know that bawl is a more specific form of the action cry)
'hearing social gathering' is the wordplay.
'hearing' shows a homophone (sound like).
'social gathering' becomes 'ball' (ball is a kind of social gathering).
'ball' is a homophone of 'BAWL'.
'on' is the link.
(Other definitions for bawl that I've seen before include "Cry loudly" , "Shout or cry noisily" , "Loudly greet" , "Loud cry" , "Loud shout" .)