Opening line in popular film (5)
I believe the answer is:
inlet
'opening' is the definition.
(I've seen this before)
'line in popular film' is the wordplay.
'line' becomes 'l'.
'in' indicates putting letters inside.
'popular' becomes 'in'.
'film' becomes 'et'.
'in'+'et'='inet'
'l' put inside 'inet' is 'INLET'.
(Other definitions for inlet that I've seen before include "Gulf" , "bit of a lake here?" , "Entry; arm of the sea" , "Fiord" , "Bay or recess" .)