Clue: In effect, leads from Tristan and Isolde very emotional after a Wagner piece? (9)
Answer: OPERATIVE
Explanation: Definition: In effect
definition: "in effect" in the sense of 'up and running' 
leading letters from T-ristan and I-solde V-ery E-motional, after OPERA="a Wagner piece"
 
*Tristan and Isolde* is a Wagner opera