Break in school is marvellous to begin with (6)
I believe the answer is:
schism
'break' is the definition.
(I've seen this in another clue)
'school is marvellous to begin with' is the wordplay.
'school' becomes 'sch' (common abbreviation).
'to begin with' indicates taking the first letters.
The initial letter of 'marvellous' is 'm'.
'sch'+'is'+'m'='SCHISM'
'in' acts as a link.
(Other definitions for schism that I've seen before include "Formal separation of one church into two" , "Split in church unity" , "Splinter group" , "Formal division of a church into two" , "Split in religious body over differences in beliefs" .)