Mouth actor's role in reverse (4)
I believe the answer is:
trap
'mouth actor's' is the definition.
'trap' can be an answer for 'mouth' (informal term for mouth). I'm not sure about the 'actor's' bit.
'role in reverse' is the wordplay.
'role' becomes 'part' (eg a part or role in a film).
'in reverse' says the letters should be written backwards.
'part' back-to-front is 'TRAP'.
Can you help me to learn more?
(Other definitions for trap that I've seen before include "Snare, ruse" , "Part of a snare" , "Ambush" , "Device to catch prey" , "Cakehole (slang)" .)