A film for everyone in the round is a wonderful thing (5)
I believe the answer is:
beaut
'a wonderful thing' is the definition.
(something beautiful)
'a film for everyone in the round' is the wordplay.
'a film for everyone' becomes 'u' (British film grading).
'in' means one lot of letters goes inside another.
'the round' becomes 'beat' (synonyms).
'u' put inside 'beat' is 'BEAUT'.
'is' is the link.
(Other definitions for beaut that I've seen before include "fine thing" , "cracker" , "16 down's fine or attractive example of something" .)