Almost entirely involved in film and sound (5)
I believe the answer is:
valid
'sound' is the definition.
('valid' can be similar in meaning to 'sound')
'almost entirely involved in film' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'entirely' becomes 'all' (I've seen this before).
'involved in' is an insertion indicator.
'film' becomes 'vid' (**).
'all' with its last letter taken off is 'al'.
'al' placed inside 'vid' is 'VALID'.
'and' is the link.
(Other definitions for valid that I've seen before include "Well-grounded" , "Based on reason or law" , "Legally accepted or logical" , "Sound and logical, also legal" , "Reasonable" .)