Itch to join club keeping old ship’s vessel (8)
I believe the answer is:
longboat
'ship's vessel' is the definition.
(boat launched from another ship)
'itch to join club keeping old' is the wordplay.
'itch' becomes 'long'.
'to join' is a charade indicator (letters next to each other).
'club' becomes 'bat' (as in sporting equipment).
'keeping' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bat' placed around 'o' is 'boat'.
'long'+'boat'='LONGBOAT'
(Other definitions for longboat that I've seen before include "one attacking from the north" , "Vessel used by the Vikings" , "Viking transport" , "Viking ship" , "one getting a row?" .)