Is a brat to book film? (4,2)
I believe the answer is:
acts up
'is a brat' is the definition.
(I've seen this in another clue)
'book film?' is the wordplay.
'book' becomes 'acts' (book of the Bible).
'film?' becomes 'Up' (Pixar film).
'acts'+'up'='ACTS UP'
'to' is the link.
(Other definitions for acts up that I've seen before include "Causes trouble or misbehaves" , "Malfunctions" , "Fails to function properly or behaves badly" , "Behaves badly (colloq.)" , "Carries on" .)