Mouth, bit to the left (4)
I believe the answer is:
trap
'mouth' is the definition.
(informal term for mouth)
'bit to the left' is the wordplay.
'bit' becomes 'part' (I've seen this in other clues).
'to the left' is a reversal indicator (reading from right to left).
'part' in reverse letter order is 'TRAP'.
(Other definitions for trap that I've seen before include "Driver's obstacle" , "Snare; pony carriage" , "Lure, snare" , "Pitfall" , "Lock" .)