Say something to curb Left's conduct (11)
I believe the answer is:
comportment
'conduct' is the definition.
(I know that comportment is a type of manner)
'say something to curb left's' is the wordplay.
'say something' becomes 'comment'.
'to curb' means one lot of letters goes inside another (curb can mean to restrain or contain).
'left' becomes 'port' (left side of a ship - opposite of starboard).
'comment' enclosing 'port' is 'COMPORTMENT'.
(Other definitions for comportment that I've seen before include "Conduct" , "personal manner" , "good behaviour" .)