School is beginning to mend split (6)
I believe the answer is:
schism
'split' is the definition.
('schism' can be a synonym of 'split')
'school is beginning to mend' is the wordplay.
'school' becomes 'sch'.
'beginning to' says to take the initial letters.
The initial letter of 'mend' is 'm'.
'sch'+'is'+'m'='SCHISM'
(Other definitions for schism that I've seen before include "Split in church unity" , "Formal division of a church into two" , "Breach within a church organisation" , "Belief-led division" , "break" .)