Vegetarians' innocent years? (5,4)
I believe the answer is:
salad days
'vegetarians' innocent years?' is the definition.
The definition suggests a plural noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for salad days that I've seen before include "Period of youth and inexperience - ate lettuce dishes?" , "Youth; heyday" , "musical" , "Period of youthful inexperience" , "Green term" .)