Joker breaks into school for change (6)
I believe the answer is:
switch
'for change' is the definition.
(switch is a kind of change)
'joker breaks into school' is the wordplay.
'joker' becomes 'wit' (a witty person).
'breaks into' is an insertion indicator.
'school' becomes 'sch' (abbreviation).
'wit' placed into 'sch' is 'SWITCH'.
(Other definitions for switch that I've seen before include "Control for making or breaking connections" , "Sudden change; a rod" , "On/off device" , "birch" , "electrical apparatus" .)