Sing about Italy's top island wine (7)
I believe the answer is:
chianti
'wine' is the definition.
(Chianti is an example)
'sing about italy's top island' is the wordplay.
'sing' becomes 'chant' (chanting is a kind of singing).
'about' means one lot of letters goes inside another.
'top' says to take the initial letters (in a down clue, the first letter is top).
'island' becomes 'i' (geographical abbreviation).
The initial letter of 'italys' is 'i'.
'chant' enclosing 'i' is 'chiant'.
'chiant'+'i'='CHIANTI'
(Other definitions for chianti that I've seen before include "Dry red Italian wine" , "Italian offering" , "Wine produced in Tuscany" , "European wine" , "I can hit for popular Italian wine" .)