Open flat-bottomed boat (4)
I believe the answer is:
punt
'open flat-bottomed boat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for punt that I've seen before include "Pole-propelled boat" , "Boat with pole - bet" , "Flat boat for Irish money" , "Former currency of Ireland" , "Flat-bottomed poled boat" .)