Also trim pieces composed by person aiming to regulate others' behaviour (8)
I believe the answer is:
moralist
'person aiming to regulate others' behaviour' is the definition.
I can't judge whether this defines the answer.
'also trim pieces' is the wordplay.
'pieces' indicates anagramming the letters (the same pieces in a new order).
'also'+'trim'='alsotrim'
'alsotrim' with letters rearranged gives 'MORALIST'.
'composed by' is the link.
Can you help me to learn more?
(Other definitions for moralist that I've seen before include "Good teacher" , "someone drawing lessons" , "preacher" , "Disciplinarian" , "Ail storm (anag)" .)