After lunch, vegetarian may be in high spirits (4,2,5)
I believe the answer is:
full of beans
'in high spirits' is the definition.
(I've seen this before)
'after lunch vegetarian may be' is the wordplay.
I cannot quite understand how this works, but
'be' is found in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for full of beans that I've seen before include "Might maracas be" , "like a cowboy after a meal on the range?" , "Pods may be" , "Very lively and cheerful" , "after feast in seedy restaurant?" .)