Millions hanging about to enjoy festival (5,4)
I believe the answer is:
mardi gras
'festival' is the definition.
(Mardi Gras is an example)
'millions hanging about to enjoy' is the wordplay.
'millions' becomes 'm' (abbreviation).
'hanging' becomes 'arras' (arras is a kind of hanging).
'about' means one lot of letters goes inside another.
'to enjoy' becomes 'dig' (to dig can mean to appreciate or 'get').
'arras' enclosing 'dig' is 'ardigras'.
'm'+'ardigras'='MARDI GRAS'
(Other definitions for mardi gras that I've seen before include "festive occasion" , "Shrove Tuesday celebration" , "The lead up to fast" , "Shrove Tuesday festival" , "Time for carnival" .)