Social set's endless boast (4)
I believe the answer is:
crow
'boast' is the definition.
(to crow about something is to boast about it)
'social set's endless' is the wordplay.
'social set' becomes 'crowd' (I've seen this before).
'endless' means to remove the last letter.
'crowd' with its last letter taken off is 'CROW'.
(Other definitions for crow that I've seen before include "One commonly stoned" , "Ugly old woman" , "Brave" , "Bird in 1's poems" , "'As the - - flies'" .)