Stravinsky's vigorous piece (4)
I believe the answer is:
igor
'stravinsky's' is the definition.
(as in the composer Igor Stravinsky)
'vigorous piece' is the wordplay.
'piece' says the answer is hidden in the clue.
'IGOR' can be found hidden inside 'vigorous'.
(Other definitions for igor that I've seen before include "Prince --, opera by Borodin" , "Prince in Borodin's opera" , "Russian boy" , "Prince of grand opera" , "-- Stravinsky, composer" .)