Scottish king finally boarded a French vessel (6)
I believe the answer is:
duncan
'scottish king' is the definition.
(I've seen this before)
'finally boarded a french vessel' is the wordplay.
'finally' suggests the final letters.
'a french' becomes 'un' ('a' in French).
'vessel' becomes 'can' (I've seen this before).
The last letter of 'boarded' is 'd'.
'd'+'un'+'can'='DUNCAN'
(Other definitions for duncan that I've seen before include "King murdered by ambitious Macbeth" , "Man's name" , "innovative dancer" , "Scottish king" , "Isadora ..., pioneer of modern dance" .)