Sauce boat finished (7)
I believe the answer is:
ketchup
'sauce' is the definition.
(I've seen this before)
'boat finished' is the wordplay.
'boat' becomes 'ketch' (ketch is a kind of sailing vessel).
'finished' becomes 'up' ('up' can be similar in meaning to 'finished').
'ketch'+'up'='KETCHUP'
(Other definitions for ketchup that I've seen before include "Sauce made largely of tomatoes" , "Red sauce" , "Spicy tomato sauce" , "Thick sauce, with tomatoes" , "Relish" .)