In class, almost all is casual (8)
I believe the answer is:
informal
'casual' is the definition.
(similar in meaning)
'in class almost all' is the wordplay.
'class' becomes 'form class' (form class is a kind of class).
'almost' means to remove the last letter (most of the word but not all of it).
'all' with its final letter removed is 'al'.
'in'+'form'+'al'='INFORMAL'
'is' acts as a link.
(Other definitions for informal that I've seen before include "Casual or unofficial" , "If normal, be casual and without ceremony" , "Casual and unceremonious" , "Unofficial, relaxed" , "Casual, free and easy" .)