Underwent an experience again, full of energy and happy (8)
I believe the answer is:
relieved
'underwent' is the definition.
Although both the answer and definition are past participle verbs, I don't see how they can define each other.
'experience again full of energy and happy' is the wordplay.
'experience again' becomes 'relive' ('live' can be a synonym for 'experience' so perhaps 'relive' is OK for 'experience again').
'full of' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'and' is a charade indicator (letters next to each other).
'happy' becomes 'd' (I can't justify this - if you can you should give a lot more credence to this answer).
'relive' going around 'e' is 'relieve'.
'relieve'+'d'='RELIEVED'
'an' is the link.
Can you help me to learn more?
(Other definitions for relieved that I've seen before include "Alleviated (pain)" , "siege is broken?" , "No longer distressed, reassured" , "Removed pain or anxiety" , "Freed by a substitute? - that relaxed the tension" .)