Vulgar book about second public gathering (5,3)
I believe the answer is:
flash mob
'public gathering' is the definition.
'vulgar book about second' is the wordplay.
'vulgar' becomes 'flash' ('flash' can be a synonym of 'vulgar'**).
'book' becomes 'b'.
'about' means one lot of letters go next to another.
'second' becomes 'mo' (both are short periods of time).
'b' put after 'mo' is 'mob'.
'flash'+'mob'='FLASH MOB'
(Other definitions for flash mob that I've seen before include "Unexpected gathering" , "Sudden mass of people" , "streakers?" , "Ad hoc group assembled to do something unusual in a public place" .)