Drunkard in Mozart opera (5)
I believe the answer is:
toper
'drunkard' is the definition.
(I've seen this before)
'in mozart opera' is the wordplay.
'in' says the answer is hidden in the clue.
'TOPER' can be found hidden inside 'mozart opera'.
(Other definitions for toper that I've seen before include "Trope for habitual drinker" , "One who drinks to excess" , "Serious drinker" , "Someone with a drink problem" , "Regular drinker" .)