Song in simple Venetian tune (9)
I believe the answer is:
barcarole
'venetian tune' is the definition.
'song in simple' is the wordplay.
'song' becomes 'carol' (carol is a kind of song**).
'in' is an insertion indicator.
'simple' becomes 'bare' ('bare' can be a synonym of 'simple').
'carol' inserted into 'bare' is 'BARCAROLE'.
(Other definitions for barcarole that I've seen before include "music of Venice" , "Gondolier's song" , "Venetian boat song" , "Venetian boating song" , "Italian song" .)