Slow learner gets letter wrong way round (4)
I believe the answer is:
late
'slow' is the definition.
(thesaurus)
'learner gets letter wrong way round' is the wordplay.
'learner' becomes 'l'.
'gets' is a charade indicator (letters next to each other).
'letter' becomes 'a'.
'wrong way' becomes 'et' (I am not sure about this - if you are sure you should believe this answer much more).
'round' says the letters should be written backwards.
'et' back-to-front is 'te'.
'l'+'a'+'te'='LATE'
(Other definitions for late that I've seen before include "Still to come - already gone!" , "after the sell-by date?" , "yet to come" , "Departed - behind time" , "Not yet arrived - departed" .)