Reportedly true length of film (4)
I believe the answer is:
reel
'length of film' is the definition.
(a reel is a length of film)
'reportedly true' is the wordplay.
'reportedly' shows a homophone (sound like).
'true' becomes 'real' (synonyms).
'real' is a homophone of 'REEL'.
(Other definitions for reel that I've seen before include "Lively dance and angler's spool" , "Dance - bobbin - stagger" , "Dance; feel giddy" , "Vigorous dance" , "Stagger about in lively dance" .)