Darn idiot in school (6)
I believe the answer is:
stitch
'darn' is the definition.
(I've seen this before)
'idiot in school' is the wordplay.
'idiot' becomes 'tit' (both can mean a stupid person).
'in' indicates putting letters inside.
'school' becomes 'sch' (abbreviation).
'tit' inserted inside 'sch' is 'STITCH'.
(Other definitions for stitch that I've seen before include "in time one could save many" , "closer" , "something seen in the needlework class" , "Sort of pain" , "One in time saves nine" .)