Mastic's moulded, not in the right part (7)
I believe the answer is:
miscast
'not in the right part' is the definition.
I can't tell whether this definition defines the answer.
'mastic's moulded' is the wordplay.
'moulded' is an anagram indicator.
'mastics' is an anagram of 'MISCAST'.
Can you help me to learn more?
(Other definitions for miscast that I've seen before include "Of actor, assigned to unsuitable role" , "Give an unsuitable role" , "Of actor, unsuited to a role" , "Given an inappropriate role" , "in the wrong film?" .)