Mastic's moulded, not in the right part (7)
I believe the answer is:
Here is my best explanation:
'not in the right part' is the definition.
I can't tell whether this definition defines the answer.
'mastic's moulded' is the subsidiary indication.
'moulded' indicates anagramming the letters.
'mastics' with letters rearranged gives 'miscast'.
(Other definitions for miscast that I've seen before include "Give an unsuitable role", "Of actor, assigned to unsuitable role", "Given an unsuitable role", "Of actor, unsuited to a role", "Given an inappropriate role", "in the wrong film?", "Given an unsuitable part", "given unsuitable role", "Assigned to an unsuitable role" and "part poorly chosen".)