Ignorant learner one marked externally (9)
I believe the answer is:
oblivious
'ignorant' is the definition.
(thesaurus)
'learner one marked externally' is the wordplay.
'learner' becomes 'l'.
'one' becomes 'i' (Roman numeral).
'marked' becomes 'obvious' ('obvious' can be similar in meaning to 'marked'**).
'externally' indicates putting letters inside (some letters go on the outside of others).
'l'+'i'='li'
'li' put inside 'obvious' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "Heedless" , "Not noticing" , "In ignorance" , "like Lethe?" , "Totally unaware" .)