Sweet cast for funny film (7,3)
I believe the answer is:
custard pie
'funny film' is the definition.
The definition suggests a singular noun which matches the answer.
'sweet cast for' is the wordplay.
'sweet' becomes 'custard' (I've seen this before).
'cast for' becomes 'pie' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'custard'+'pie'='CUSTARD-PIE'
Can you help me to learn more?
(Other definitions for custard pie that I've seen before include "What featured in comedy" , "Item thrown in slapstick comedy" , "Clown's dessert?" , "Slapstick missile" , "But is one thrown" .)