Song line noted in plain boating tune (10)
I believe the answer is:
Here is my best explanation:
'boating tune' is the definition.
(I've seen this before)
'song line noted in plain' is the subsidiary indication.
I cannot quite understand how this works, but
'song' could be 'carol' (carol is a kind of song) and 'carol' is present in the answer.
'line' could be 'bar' (bar line is a kind of line) and 'bar' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for barcarolle that I've seen before include "musical entertainment", "Gondolier's song", "Song with liquid associations", "boating tune" and "musical piece".)