Short piece of music you perform, ultimately, least (7)
I believe the answer is:
minimum
'least' is the definition.
(I've seen this before)
'short piece of music you perform ultimately' is the wordplay.
'short' becomes 'mini' ('mini' can be similar in meaning to 'short'**).
'piece of' suggests taking the first letters.
'ultimately' says to take the final letters.
The first letter of 'music' is 'm'.
The last letters of 'you perform' are 'um'.
'mini'+'m'+'um'='MINIMUM'
(Other definitions for minimum that I've seen before include "The least of smallest amount possible" , "Least recorded value" , "Smallest possible quantity" , "Bottom" , "Lowest point" .)