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.
'cuba' becomes 'c'.
'on top' indicates taking the first letters.
'uprising' says the letters should be written backwards.
'from' becomes 'fr' (abbreviation).
'briefly united' becomes 'U' (abbreviation; 'briefly' asks for a short form).
The first letter of 'of' is 'o'.
'fr' written backwards 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" .)