Film moderates casing joint (9,4)
I believe the answer is:
watership down
'film' is the definition.
(I've seen this before)
'moderates casing joint' is the wordplay.
'moderates' becomes 'watersdown' (to water something down is to moderate it).
'casing' means one lot of letters goes inside another (I've seen this in other clues).
'joint' becomes 'hip' (hip joint is a kind of joint).
'watersdown' placed around 'hip' is 'WATERSHIP DOWN'.
(Other definitions for watership down that I've seen before include "Story about rabbits" , "Adams rabbit novel" , "Book about rabbits" , "Film" .)