Best time of youth (5,4)
I believe the answer is:
salad days
'best time of youth' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for salad days that I've seen before include "inexperienced youth" , "Green term" , "Period of youth and inexperience - ate lettuce dishes?" , "Time of youth and inexperience" , "Period of youthful inexperience" .)