Youngster, a learner unqualified (5)
I believe the answer is:
total
'unqualified' is the definition.
('total' can be a synonym of 'unqualified')
'youngster a learner' is the wordplay.
'youngster' becomes 'tot' (tot is a kind of youngster**).
'learner' becomes 'l'.
'tot'+'a'+'l'='TOTAL'
(Other definitions for total that I've seen before include "Not qualified" , "wreck" , "Complete, absolute" , "Reckon" , "outright" .)