Somehow he's best, keen, full of energy — wonderful person (3,4,5)
I believe the answer is:
the bees knees
'wonderful person' is the definition.
(informal term for an excellent person or thing)
'somehow he's best keen full of energy' is the wordplay.
'somehow' indicates an anagram.
'full of' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'hes'+'best'+'keen'='hesbestkeen'
'hesbestkeen' anagrammed gives 'thebeesknes'.
'thebeesknes' enclosing 'e' is 'THE BEES KNEES'.
(Other definitions for the bees knees that I've seen before include "Excellent thing" , "Outstandingly good thing" , "joints used to crawl with insects?" , "Ideal person, event" , "Just the very best" .)