Penny appears in old-time musical work (5)
I believe the answer is:
opera
'musical work' is the definition.
(operas are musical works)
'penny appears in old-time' is the wordplay.
'penny' becomes 'p' (short for pence).
'appears in' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'time' becomes 'era' (era is a kind of time).
'o'+'era'='oera'
'p' inserted inside 'oera' is 'OPERA'.
(Other definitions for opera that I've seen before include "eg Peter Grimes" , "Eg, Der Rosenkavalier" , "works in Latin" , "La Boheme or Rigoletto" , "Billy Budd, for instance" .)