Comprehensive classes getting large (7)
I believe the answer is:
general
'comprehensive' is the definition.
('general' can be similar in meaning to 'comprehensive')
'classes getting large' is the wordplay.
'classes' becomes 'genera' (I've seen this before).
'getting' means one lot of letters go next to another.
'large' becomes 'l'.
'genera'+'l'='GENERAL'
(Other definitions for general that I've seen before include "Not specialised or limited" , "Not particular or specific" , "Enlarge for senior army officer" , "Unspecific" , "Rank; normal, usual" .)