Unusual tempo to maintain one piece of music (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 to maintain one' is the wordplay.
'unusual' indicates an anagram.
'to maintain' indicates putting letters inside (maintain can mean to keep or preserve).
'tempo' is an anagram of 'tpoem'.
'tpoem' going around 'one' is 'TONE-POEM'.
(Other definitions for tone poem that I've seen before include "Work" , "Orchestral piece often on a literary theme" , "Story-based orchestral piece" .)