Scotch is a no-no for Rugby School's leader (5,2,3,4)
I believe the answer is:
knock on the head
'scotch' is the definition.
(both can mean to put an end to something)
'no-no for rugby school's leader' is the wordplay.
'no-no for rugby' becomes 'knock-on' (knocking-on is not allowed in rugby).
'school's leader' becomes 'the head' (headteacher).
'knockon'+'thehead'='KNOCK ON THE HEAD'
'is a' is the link.
(Other definitions for knock on the head that I've seen before include "tap being applied to nut" , "Stop" .)