A number of punters sing in nude (9)
I believe the answer is:
barcarole
'a number of punters' is the definition.
(song sung by gondoliers - people who use punts)
'sing in nude' is the wordplay.
'sing' becomes 'carol' (caroling is a kind of singing).
'in' is an insertion indicator.
'nude' becomes 'bare' (synonyms).
'carol' put inside 'bare' is 'BARCAROLE'.
(Other definitions for barcarole that I've seen before include "musical composition" , "Italian song" , "music of Venice" , "Gondolier's song" , "Venetian boat song" .)