Greet young'un in a new musical (5,3,4,3)
I believe the answer is:
annie get your gun
'musical' is the definition.
(annie get your gun is a musical)
'greet young'un in a new' is the wordplay.
'new' indicates anagramming the letters.
'greet'+'youngun'+'in'+'a'='greetyoungunina'
'greetyoungunina' anagrammed gives 'ANNIE GET YOUR GUN'.
(Other definitions for annie get your gun that I've seen before include "call to arms" , "musical" , "film" , "Girl instructed to defend her own" .)