Mock a closing musical piece (4)
I believe the answer is:
coda
'closing musical piece' is the definition.
(a coda appears at the end of a piece of music)
'mock a' is the wordplay.
'mock' becomes 'cod' (cod can mean false or inauthentic).
'cod'+'a'='CODA'
(Other definitions for coda that I've seen before include "Passage of piece of music added to the end" , "Final part of mus. structure" , "Musical finale" , "noted conclusion" , "Closing stage" .)