In class nearly all relaxed (8)
I believe the answer is:
informal
'relaxed' is the definition.
(informal can mean relaxed or casual)
'in class nearly all' is the wordplay.
'class' becomes 'form' (synonyms).
'nearly all' becomes 'al'.
'in'+'form'+'al'='INFORMAL'
(Other definitions for informal that I've seen before include "Casual, free and easy" , "Casual or unofficial" , "If normal, be casual and without ceremony" , "Casual, not official" , "Unofficial, relaxed" .)