Not doing well, missing class (3,2,4)
I believe the answer is:
out of form
'not doing well' is the definition.
(I've seen this before)
'missing class' is the wordplay.
'missing' becomes 'out of' (I've seen this in another clue).
'class' becomes 'form class' (synonyms).
'outof'+'form'='OUT OF FORM'
(Other definitions for out of form that I've seen before include "not playing well" , "Off school" , "In poor shape" .)