School equal to fight (4)
I believe the answer is:
spar
'fight' is the definition.
(sparring is a kind of fighting)
'school equal' is the wordplay.
'school' becomes 's'.
'equal' becomes 'par'.
's'+'par'='SPAR'
'to' acts as a link.
(Other definitions for spar that I've seen before include "Box - mineral - rod" , "Box; various minerals" , "Crystalline mineral" , "Pole to support sails, rigging etc" , "Rafter, boom" .)