Further advanced lesson regularly required (4)
I believe the answer is:
also
'further' is the definition.
(thesaurus)
'advanced lesson regularly required' is the wordplay.
'advanced' becomes 'a'.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'required' means one lot of letters go next to another.
The alternate letters of 'lesson' are 'lso'.
'a'+'lso'='ALSO'
(Other definitions for also that I've seen before include "on top" , "Furthermore" , "Likewise" , "item's rarely used now for this" , "'Besides, moreover (4)'" .)