Opening line on film: watch at the start (6)
I believe the answer is:
eyelet
'opening' is the definition.
(hole for putting a shoelace through)
'line on film watch at the start' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'on' is a charade indicator (letters next to each other).
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'watch' becomes 'eye' (eye can mean to watch or look at).
'at the start' means one lot of letters go next to another.
'l' put next to 'et' is 'let'.
'let' put after 'eye' is 'EYELET'.
(Other definitions for eyelet that I've seen before include "Shoe hole for lacing" , "Small reinforced hole" , "Fastening hole" , "peephole" , "Hole for, eg, shoe lace" .)