In effect, leads from Tristan and Isolde very emotional after a Wagner piece? (9)
I believe the answer is:
operative
'in effect' is the definition.
(similar in meaning)
'leads from tristan and isolde very emotional after a wagner piece?' is the wordplay.
I cannot quite see how this works, but
'from' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'very' could be 'v' (abbreviation) and 'v' is present in the answer.
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for operative that I've seen before include "Machinist - working" , "Worker in manufacturing" , "American detective" , "secret agent" , "Functioning; worker" .)