Head-banger's ballet?
I believe the answer is:
nutcracker
'ballet?' is the definition.
(I've seen this before)
'head-banger's' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head).
'banger' becomes 'cracker' (synonyms).
'nut'+'cracker'='NUTCRACKER'
(Other definitions for nutcracker that I've seen before include "Opener for brazils, almonds, etc" , "The --, ballet by Tchaikovsky" , "Brazil under pressure from this" , "Bird of the crow family" , "The work of Tchaikovsky" .)