Missing classes, feeling low (3,2,5)
I believe the answer is:
out of sorts
'feeling low' is the definition.
I know nothing about this answer so I can't judge whether this works.
'missing classes' is the wordplay.
'missing' becomes 'out of' (I've seen this in another clue).
'classes' becomes 'sorts' ('sort' can be a synonym of 'class').
'outof'+'sorts'='OUT OF SORTS'
Can you help me to learn more?
(Other definitions for out of sorts that I've seen before include "Lacking variety" , "Not well" , "Irritable" , "Not feeling good" , "Slightly unwell" .)