Youngster upsetting a gent 'ere (8)
I believe the answer is:
teenager
'youngster' is the definition.
(thesaurus)
'upsetting a gent 'ere' is the wordplay.
'upsetting' indicates an anagram.
'a'+'gent'+'ere'='agentere'
'agentere' with letters rearranged gives 'TEENAGER'.
(Other definitions for teenager that I've seen before include "Youngster" , "Generate (anag)" , "Adolescent" , "one that's not yet made a score?" , "one no longer twelve?" .)