Produced by one ringleader, a fortified wine (7)
I believe the answer is:
madeira
'fortified wine' is the definition.
(Madeira is an example)
'produced by one ringleader a' is the wordplay.
'produced' becomes 'made' ('make' can be a synonym of 'produce').
'by' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'ringleader' becomes 'r' (first letter of ring).
'made'+'i'+'r'+'a'='MADEIRA'
(Other definitions for madeira that I've seen before include "Sponge-cake" , "Island in the Atlantic Ocean" , "Portuguese island off northwest Africa" , "Atlantic territory" , "Fortified white wine from island off NW Africa" .)