Cuba on top of uprising from briefly united island (5)
I believe the answer is:
corfu
'island' is the definition.
(Greek island)
'cuba on top of uprising from briefly united' is the wordplay.
'on top' says to take the initial letters.
'of' becomes 'o''.
'uprising' is a reversal indicator.
'from' becomes 'fr' (abbreviation).
'briefly united' becomes 'U' (abbreviation as in Man U for Manchester United; 'briefly' asks for a short form).
The initial letter of 'cuba' is 'c'.
'fr' reversed gives 'rf'.
'c'+'o'+'rf'+'u'='CORFU'
(Other definitions for corfu that I've seen before include "Greece's 'Kerkyra'" , "Largest Ionian Island" , "Ionian holiday island" , "Greek holiday island" , "Holiday location" .)