Quantity of film showing dance (4)
I believe the answer is:
reel
I believe this clue is a double definition.
'quantity of film' is the first definition.
(I've seen this in another clue)
'dance' is the second definition.
(I know that reel is a type of social dancing)
'showing' acts as a link.
(Other definitions for reel that I've seen before include "Fishing rod attachment" , "Drum - dance - music" , "Part of a film; stagger" , "The angler pulls in his line using this" , "Part of film; dance" .)