Exchange involving humorist in school (6)
I believe the answer is:
switch
'exchange' is the definition.
(switching is a kind of exchanging)
'humorist in school' is the wordplay.
'humorist' becomes 'wit' (wit is a kind of humorist).
'in' means one lot of letters goes inside another.
'school' becomes 'sch'.
'wit' placed within 'sch' is 'SWITCH'.
'involving' is the link.
(Other definitions for switch that I've seen before include "On/off device" , "Exchange; contactor" , "Device for changing electrical connections" , "Sudden quick change" , "Get change" .)