On a plank (6)
I believe the answer is:
aboard
'on a plank' is the definition.
'on a plank' is the wordplay.
'on' says to put letters next to each other.
'plank' becomes 'board' ('board' can be a synonym of 'plank').
'a'+'board' is 'ABOARD'.
(Other definitions for aboard that I've seen before include "On (a ship/plane)" , "on a train, perhaps" , "Part of the group" , "on a ferry?" , "On (a ship, train, etc)" .)