1001 and not a young person! (5)
I believe the answer is:
minor
'a young person' is the definition.
(I've seen this before)
'1001 and not' is the wordplay.
'1001' becomes 'mi' (I've seen this in other clues).
'and not' becomes 'nor' ('nor' means 'and also not').
'mi'+'nor'='MINOR'
(Other definitions for minor that I've seen before include "Person not yet of legal age" , "Not major" , "Paltry" , "Trivial" , "Deceptively realistic (art)" .)