Idea mostly incorporated into very old film (5)
I believe the answer is:
video
'film' is the definition.
(thesaurus)
'idea mostly incorporated into very old' is the wordplay.
'mostly' means to remove the last letter.
'incorporated into' indicates putting letters inside.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'idea' with its last letter taken off is 'ide'.
'v'+'o'='vo'
'ide' going inside 'vo' is 'VIDEO'.
(Other definitions for video that I've seen before include "Film recording" , "Who killed the radio star? (5)" , "Transmitter of televised image" , "(Tape for) recorder of film etc" , "Taped film" .)