Class of thirty cleared out before tiring lesson (4)
I believe the answer is:
type
'class' is the definition.
(thesaurus)
'thirty cleared out before tiring lesson' is the wordplay.
'cleared out' means to remove the middle letters.
'before' is a charade indicator (letters next to each other).
'tiring lesson' becomes 'pe' (abbreviation for Physical Education).
'thirty' with its centre taken out is 'ty'.
'ty'+'pe'='TYPE'
'of' is the link.
(Other definitions for type that I've seen before include "Sort, kind" , "key" , "Mark" , "Print - classification" , "Thing or person of a specified kind" .)