Look up part of film (4)
I believe the answer is:
reel
'part of film' is the definition.
(as in a reel of film)
'look up' is the wordplay.
'look' becomes 'leer' (leering is a kind of looking).
'up' is a reversal indicator (in down clue: letters go upwards).
'leer' in reverse letter order is 'REEL'.
(Other definitions for reel that I've seen before include "energetic dance" , "Air" , "Lurch; spool" , "blows making one do this?" , "unwind?" .)