The headmaster was allegedly given this nickname by Ged
Greenhaulgh ('65 entry).
During a discussion on Spike's prominent proboscis in 1965, Ged came up with the name and it stuck.
From a Latin Dictionary [discovered years later - Ed ] :
Clavus -i m.
(1) a nail or spike
(2) a tiller, helm or rudder
(3) a stripe of purple on a tunic, worn broad by senators, narrow by knights