Film about day centre (5)
I believe the answer is:
midst
'centre' is the definition.
(I've seen this before)
'film about day' is the wordplay.
'film' becomes 'mist' (to film over is to mist or fog over).
'about' is an insertion indicator.
'day' becomes 'd' (abbreviation for day).
'mist' placed around 'd' is 'MIDST'.
(Other definitions for midst that I've seen before include "Among, amid" , "Where one is surrounded by other things" , "Thick (of things)" , "Blue dye; rainbow hue" , "In the middle of (archaic or literary)" .)