Understand loud social gathering hasn’t ended (6)
I believe the answer is:
fathom
'understand' is the definition.
(to fathom something is to understand it)
'loud social gathering hasn't ended' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'social gathering' becomes 'at home' (old-fashioned term for a party).
'hasn't ended' means to remove the last letter.
'athome' with its last letter taken away is 'athom'.
'f'+'athom'='FATHOM'
(Other definitions for fathom that I've seen before include "Come to understand the water depth" , "Work out" , "Understand (something) after much thought" , "Come to understand - unit of depth" , "Six feet in depth; grasp" .)