Left welcomes new support (6)
I believe the answer is:
splint
'support' is the definition.
(I've seen this before)
'left welcomes new' is the wordplay.
'left' becomes 'split' (split can mean to run away or leave).
'welcomes' is an insertion indicator (in sense of 'invites in').
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'split' enclosing 'n' is 'SPLINT'.
(Other definitions for splint that I've seen before include "Rigid device supporting a fractured bone" , "Piece or rigid material to secure broken bone" , "Stick or straight object to which broken limb is fastened" , "Rigid support for a broken bone" , "strip of wood" .)