Horse featured in problematical film (3,5)
I believe the answer is:
the sting
'problematical film' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'horse featured in' is the wordplay.
I cannot really understand how this works, but
'horse' could be 'h' (synonyms) and 'h' is located in the answer.
'in' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for the sting that I've seen is " film from 1970s".)