Bliss found in sostenuto piano piece (6)
I believe the answer is:
utopia
'bliss' is the definition.
(I've seen this before)
'in sostenuto piano piece' is the wordplay.
'piece' indicates the answer is hidden within the clue.
'UTOPIA' is hidden in the letters of 'in sostenuto piano'.
'found' acts as a link.
(Other definitions for utopia that I've seen before include "Impracticable state of perfection" , "Heaven" , "Place of ideal perfection" , "Ideal but unattainable state" , "Supposed ideal land" .)