Finished the prom piano act Verdi regularly forgot (10)
I believe the answer is:
terminated
'finished' is the definition.
('terminate' can be a synonym of 'finish')
'the prom piano act verdi regularly forgot' is the wordplay.
'regularly forgot' indicates alternate letters (letters removed at regular intervals).
'the'+'prom'+'piano'+'act'+'verdi'='theprompianoactverdi'
The alternating letters of 'theprompianoactverdi' are 'TERMINATED'.
(Other definitions for terminated that I've seen before include "Ended" , "Brought to an end" , "Stopped" , "done something with extreme prejudice" , "reached the end of the line" .)