Youngster with endless energy ate nuts (8)
I believe the answer is:
teenager
'youngster' is the definition.
(thesaurus)
'endless energy ate nuts' is the wordplay.
'endless' means to remove the last letter.
'nuts' indicates an anagram (nuts can mean crazy).
'energy' with its last letter removed is 'energ'.
'energ'+'ate'='energate'
'energate' with letters rearranged gives 'TEENAGER'.
'with' acts as a link.
(Other definitions for teenager that I've seen before include "Generate (anag)" , "Youngster" , "hormonal problem?" , "troublesome youth" , "14, perhaps" .)