Learner, one with extremely distinctive uniform (6)
I believe the answer is:
livery
'distinctive uniform' is the definition.
(I've seen this before)
'learner one with extremely' is the wordplay.
'learner' becomes 'l'.
'one' becomes 'i' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'extremely' becomes 'very' (I've seen this before).
'l'+'i'+'very'='LIVERY'
(Other definitions for livery that I've seen before include "The stabling and care of horses" , "outfit in uniform" , "group of servants" , "Uniform -- suffering from gastric distress" , "Uniform, eg of guild" .)