Musical work in prison — so long (7)
I believe the answer is:
cantata
'musical work' is the definition.
(type of musical composition)
'prison so long' is the wordplay.
'prison' becomes 'can' (the can is American slang for prison).
'so long' becomes 'ta ta' (both are ways of saying goodbye).
'can'+'tata'='CANTATA'
'in' is the link.
(Other definitions for cantata that I've seen before include "Musical composition with religious text" , "Vocal composition" , "aria" , "Setting of words to music" , "Vocal-orchestral piece" .)