Sauce boat enlarged (7)
I believe the answer is:
ketchup
'sauce' is the definition.
(ketchup is a type of sauce)
'boat enlarged' is the wordplay.
'boat' becomes 'ketch' (ketch is a kind of sailing vessel).
'enlarged' becomes 'up' (gone up in size).
'ketch'+'up'='KETCHUP'
(Other definitions for ketchup that I've seen before include "Red sauce" , "Spicy tomato sauce" , "Relish" , "Condiment" , "Thick sauce, with tomatoes" .)