Song line noted in plain boating tune (10)
I believe the answer is:
barcarolle
'boating tune' is the definition.
(I've seen this before)
'song line noted in plain' is the wordplay.
I cannot quite see 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 located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for barcarolle that I've seen before include "musical entertainment" , "Gondolier's song" , "Song with liquid associations" , "boating tune" , "musical piece" .)