Like a boat on the canal (6)
I believe the answer is:
narrow
'the canal' is the definition.
(thesaurus)
'like a boat' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'boat' could be 'row' (rowing is a kind of boating) and 'row' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'on' acts as a link.
Can you help me to learn more?
(Other definitions for narrow that I've seen before include "Conservative" , "Vessel designed for canals" , "Of limited width" , "Thin; taper" , "Keen" .)