Appear in musical (4)
I believe the answer is:
show
I believe this clue is a double definition.
'appear' is the first definition.
(showing is a kind of appearing)
'musical' is the second definition.
(thesaurus)
'in' acts as a link.
(Other definitions for show that I've seen before include "Demonstrate; spectacle" , "Labour's evidence for starting" , "Prove; exhibit" , "event" , "Performance" .)