Members of Balliol initially supporting gaudy meet for a purpose (5,3)
I believe the answer is:
flash mob
'for a purpose' is the definition.
I can't tell whether this defines the answer.
'members of balliol initially supporting gaudy' is the wordplay.
'initially' suggests taking the first letters.
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'gaudy' becomes 'flash' (synonyms).
The first letters of 'members of balliol' is 'mob'.
'mob' put after 'flash' is 'FLASH-MOB'.
'meet' acts as a link.
Can you help me to learn more?
(Other definitions for flash mob that I've seen before include "Sudden mass of people" , "streakers?" , "Ad hoc group assembled to do something unusual in a public place" , "Unexpected gathering" .)