Unexpected gathering of streakers? (5,3)
I believe the answer is:
flash mob
I believe this clue is a double definition.
'unexpected gathering' is the first definition.
(I've seen this before)
'streakers?' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for flash mob that I've seen before include "Sudden mass of people" , "Ad hoc group assembled to do something unusual in a public place" , "streakers?" , "Unexpected gathering" .)