A plank on the ship (6)
I believe the answer is:
aboard
'on the ship' is the definition.
(I've seen this before)
'a plank' is the wordplay.
'plank' becomes 'board' (synonyms).
'a'+'board'='ABOARD'
(Other definitions for aboard that I've seen before include "in a car" , "ready to take off" , "Alongside" , "on a train, perhaps" , "Joined for mutual benefit" .)