Boat taking 500 in a sort of lagoon (7)
I believe the answer is:
gondola
'boat' is the definition.
(gondola is a kind of boat)
'500 in a sort of lagoon' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'in' is an insertion indicator.
'a sort of' indicates an anagram.
'lagoon' with letters rearranged gives 'gonola'.
'd' going into 'gonola' is 'GONDOLA'.
'taking' is the link.
(Other definitions for gondola that I've seen before include "Boat on Venetian canals" , "Traditionally used on Venetian canals" , "Venetian transport" , "Venice craft" , "Goon lad (anag.)" .)