Youth, ten, eager to develop (8)
I believe the answer is:
teenager
'youth' is the definition.
(I've seen this before)
'ten eager to develop' is the wordplay.
'to develop' is an anagram indicator.
'ten'+'eager'='teneager'
'teneager' with letters rearranged gives 'TEENAGER'.
(Other definitions for teenager that I've seen before include "14, perhaps" , "youth, perhaps" , "hormonal problem?" , "Generate (anag)" , "troublesome youth" .)