On reflection, it’s a sparkling wine (4)
I believe the answer is:
asti
'sparkling wine' is the definition.
(I've seen this before)
'on reflection it's a' is the wordplay.
'on reflection' says the letters should be written in reverse.
'its'+'a'='itsa'
'itsa' written backwards gives 'ASTI'.
(Other definitions for asti that I've seen before include "Capital of a north-west Italian province, known for its sparking wine" , "Bubbly wine" , "Source of fizz" , "town in Italy" , "Italian white wine" .)