Plan of school heads in early mainstream education (6)
I believe the answer is:
scheme
'plan' is the definition.
(I know that scheme is a type of plan)
'school heads in early mainstream education' is the wordplay.
'school' becomes 'sch' (abbreviation).
'heads in' indicates taking the first letters (I've seen 'head in' mean this).
The initial letters of 'early mainstream education' is 'eme'.
'sch'+'eme'='SCHEME'
'of' is the link.
(Other definitions for scheme that I've seen before include "Machination" , "Plan (maybe best laid?)" , "programme" , "Plan, ruse" , "Conive, intrigue" .)