Greek island my favourite uncle originally used (5)
I believe the answer is:
corfu
'greek island' is the definition.
(Corfu is a Greek island)
'my favourite uncle originally used' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'originally used' indicates taking the first letters.
The initial letters of 'favourite uncle' is 'fu'.
'cor'+'fu'='CORFU'
(Other definitions for corfu that I've seen before include "Greece's 'Kerkyra'" , "Largest Ionian Island" , "Gk. island, opposite Albania" , "Greek holiday island" , "Greek island in the Ionian Sea" .)