Learner being kept in that is least likely to make an effort (6)
I believe the answer is:
idlest
'least likely to make an effort' is the definition.
(someone idle does not make an effort)
'learner being kept in that is' is the wordplay.
'learner' becomes 'l'.
'being kept in' indicates putting letters inside.
'that is' becomes 'idest' ('that is' in Latin).
'l' inserted inside 'idest' is 'IDLEST'.
(Other definitions for idlest that I've seen before include "Least hard-working" , "least fond of Labour?" , "Most lazy" , "most unused" , "Least diligent" .)