Course right for vessel (5)
I believe the answer is:
'for vessel' is the definition.
(liner is a kind of vessel)
'course right' is the wordplay.
'course' becomes 'line' (synonyms).
'right' becomes 'r' (common abbreviation).
(Other definitions for liner that I've seen before include "Large passenger-ship" , "Pleasure cruiser" , "Cosmetic item - Titanic, for example" , "Passenger transport" , "that's found in the kitchen bin, perhaps" .)