In class a pupil relaxed (8)
I believe the answer is:
informal
'relaxed' is the definition.
(informal can mean relaxed or casual)
'in class a pupil' is the wordplay.
'class' becomes 'form class' (synonyms).
'pupil' becomes 'L' (as in a learner driver with L-plates on their car).
'in'+'form'+'a'+'l'='INFORMAL'
(Other definitions for informal that I've seen before include "Cosy -- loose" , "Casual and unceremonious" , "If normal, be casual and without ceremony" , "Casual, free and easy" , "Casual, not official" .)