Explosive reagent claims eastern youngster (8)
I believe the answer is:
teenager
'youngster' is the definition.
(I know that teenager is a type of youngster)
'explosive reagent claims eastern' is the wordplay.
'explosive' indicates an anagram.
'claims' means one lot of letters goes inside another.
'eastern' becomes 'e'.
'reagent' anagrammed gives 'teenagr'.
'teenagr' placed around 'e' is 'TEENAGER'.
(Other definitions for teenager that I've seen before include "hormonal problem?" , "one no longer twelve?" , "one that's not yet made a score?" , "troublesome youth" , "14, perhaps" .)