Vessel not exactly broad in the beam? (6,4)
I believe the answer is:
narrow boat
'vessel' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for narrow boat that I've seen before include "Canal craft" , "Canal barge" , "Vessel designed for canals" , "Traditional canal transport" , "specialised vessel" .)