Casual scoundrel about to break into school (7)
I believe the answer is:
scratch
'casual' is the definition.
(thesaurus)
'scoundrel about to break into school' is the wordplay.
'scoundrel' becomes 'rat' (both can mean a dishonourable person).
'about' becomes 'c' (circa).
'to break into' indicates putting letters inside (inserted letters break into the word).
'school' becomes 'sch' (abbreviation).
'rat'+'c'='ratc'
'ratc' inserted into 'sch' is 'SCRATCH'.
(Other definitions for scratch that I've seen before include "Old Nick" , "Handicap where expected to make" , "Withdraw from (competition)" , "Improvised" , "given no advantage" .)