Ruler thwacking student across rump, originally (4)
I believe the answer is:
tsar
'ruler' is the definition.
(historical term for Russian king)
'thwacking student across rump originally' is the wordplay.
'originally' suggests taking the first letters.
The first letters of 'thwacking student across rump' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Potentate" , "Supremo" , "Russian leader" , "Absolute ruler" , "Despot" .)