Piece of music with unusual tempo entertaining one (4,4)
I believe the answer is:
tone poem
'piece of music' is the definition.
(tone poem is a kind of piece of music)
'unusual tempo entertaining one' is the wordplay.
'unusual' indicates an anagram.
'entertaining' indicates putting letters inside.
'tempo' with letters rearranged gives 'tpoem'.
'tpoem' going around 'one' is 'TONE-POEM'.
'with' acts as a link.
(Other definitions for tone poem that I've seen before include "Orchestral piece often on a literary theme" , "Story-based orchestral piece" , "Work" .)