In class a learner is casual (8)
I believe the answer is:
informal
'casual' is the definition.
(similar in meaning)
'in class a learner' is the wordplay.
'class' becomes 'form' (form class is a kind of class).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in'+'form'+'a'+'l'='INFORMAL'
'is' is the link.
(Other definitions for informal that I've seen before include "Casual or unofficial" , "Casual, free and easy" , "Casual, not official" , "Cosy -- loose" , "Unofficial, relaxed" .)