Longboats on inland waterways (6)
I believe the answer is:
barges
'longboats on inland waterways' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for barges that I've seen before include "Long boats on inland waterways" , "Moves clumsily in canal boats" , "Broad flat-bottomed boats" , "Inland waterway longboats" , "Inland waterway boats" .)