Idiot in school gets pain from exercising (6)
I believe the answer is:
stitch
'pain from exercising' is the definition.
'stitch' can be an answer for 'pain' (pain in the body). I am not sure about the 'from exercising' bit.
'idiot in school' is the wordplay.
'idiot' becomes 'tit' (both can mean a stupid person).
'in' means one lot of letters goes inside another.
'school' becomes 'sch' (common abbreviation).
'tit' placed inside 'sch' is 'STITCH'.
'gets' acts as a link.
Can you help me to learn more?
(Other definitions for stitch that I've seen before include "One in time can save nine, they say" , "Sort of pain" , "Darn" , "''A . . . . . . in time saves nine'' (6)" , "Application of needle produces this" .)