Sullen kid desperately needing training (9)
I believe the answer is:
unskilled
'needing training' is the definition.
(I've seen this before)
'sullen kid desperately' is the wordplay.
'desperately' indicates an anagram.
'sullen'+'kid'='sullenkid'
'sullenkid' is an anagram of 'UNSKILLED'.
(Other definitions for unskilled that I've seen before include "Inexperienced" , "Requiring no special training" , "Incompetent" , "Lacking professional expertise" , "Clumsy" .)