Fifties' musical a beginner put on in States (5,4)
I believe the answer is:
Here is my best explanation:
'fifties' musical' is the definition.
I don't understand how the remainder of the clue works.
(Other definitions for salad days that I've seen before include "Carefree youth", "Green term", "Period of youth and inexperience - on lettuce diet?", "Time for a vegetarian diet", "musical", "Youth; heyday", "Period of youth and inexperience - ate lettuce dishes?", "Best time of youth", "Time of youth and inexperience" and "Period of youthful inexperience".)