Fifties' musical a beginner put on in States (5,4)

I believe the answer is:

salad days

'fifties' musical' is the definition.

I can't explain the remainder of the clue.

(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" .)

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app