Crossword Genius

Happy after a vegetarian meal? (4,2,5)

Ross

I believe the answer is:

full of beans

I'm a little stuck... Click here to teach me more about this clue!

'vegetarian meal?' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'happy after a' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for full of beans that I've seen before include "Energised" , "effervescent" , "Eager and alert; full of high spirits" , "Might maracas be" , "like a cowboy after a meal on the range?" .)

I've seen this clue in the Sydney Morning Herald.
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