Universal fur is functional (6)
I believe the answer is:
usable
'functional' is the definition.
(having a use)
'universal fur' is the wordplay.
'universal' becomes 'U' (film certificate).
'fur' becomes 'sable' (sable is a kind of fur).
'u'+'sable'='USABLE'
'is' is the link.
(Other definitions for usable that I've seen before include "In working order" , "Functioning" , "Fit for employment" , "Serviceable" , "Like something functional" .)