Leave old rein out for oarsman (9)
I believe the answer is:
gondolier
'for oarsman' is the definition.
(thesaurus)
'leave old rein out' is the wordplay.
'leave' becomes 'go' (to go is to depart or leave).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'old'+'rein'='oldrein'
'oldrein' anagrammed gives 'ndolier'.
'go'+'ndolier'='GONDOLIER'
(Other definitions for gondolier that I've seen before include "Craftsman" , "Venetian boater" , "Venetian boatman" , "Venetian waterman" , "to leave Red Lion disorientated" .)