Pine and willow casing old vessel (8)
I believe the answer is:
longboat
'vessel' is the definition.
(longboat is a kind of vessel)
'pine and willow casing old' is the wordplay.
'pine' becomes 'long' (to pine for something is to long for it).
'and' means one lot of letters go next to another.
'willow' becomes 'bat' (willow can mean a cricket bat).
'casing' indicates putting letters inside (I've seen this in other clues).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bat' going around 'o' is 'boat'.
'long'+'boat'='LONGBOAT'
(Other definitions for longboat that I've seen before include "Vessel used by the Vikings" , "one getting a row?" , "Viking warship" , "Viking ship" , "Ship's largest vessel" .)