People gathering quickly mostly shamble off endlessly in disarray (5,3)
I believe the answer is:
flash mob
'people gathering quickly' is the definition.
(group of people who assemble quickly)
'mostly shamble off endlessly in disarray' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'endlessly' means to remove the last letter.
'in disarray' is an anagram indicator.
'shamble' with its last letter taken away is 'shambl'.
'off' with its final letter taken off is 'of'.
'shambl'+'of'='shamblof'
'shamblof' is an anagram of 'FLASH-MOB'.
(Other definitions for flash mob that I've seen before include "Unexpected gathering" , "streakers?" , "Sudden mass of people" , "Ad hoc group assembled to do something unusual in a public place" .)