A learner joins class that's strict (6)
I believe the answer is:
formal
'strict' is the definition.
(thesaurus)
'a learner joins class' is the wordplay.
'learner' becomes 'l'.
'joins' is a charade indicator (letters next to each other).
'class' becomes 'form' (form class is a kind of class**).
'a'+'l'='al'
'al' after 'form' is 'FORMAL'.
'that's' is the link.
(Other definitions for formal that I've seen before include "Conforming to accepted rules or customs" , "Cermonial, solemn" , "Unbending" , "Ceremonial, solemn" , "Ceremonial; ordered" .)