Piece of music to some extent in one tempo (5)
I believe the answer is:
nonet
'piece of music' is the definition.
(I've seen this before)
'to some extent in one tempo' is the wordplay.
'to some extent' indicates a hidden word (I've seen 'extent of' mean this (I've seen this in other clues)).
'NONET' can be found hidden inside 'in one tempo'.
(Other definitions for nonet that I've seen before include "Tonne (anag.); music for a group" , "Work for nine players" , "Composition for nine instruments" , "group composition" , "musical piece" .)