Greek punishment backed by one Conservative (5)
I believe the answer is:
doric
'greek' is the definition.
(I know that Doric is a type of Ancient Greek)
'punishment backed by one conservative' is the wordplay.
'punishment' becomes 'rod' (stick used for corporal punishment).
'backed' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'conservative' becomes 'C' (UK political abbreviation).
'rod' written backwards gives 'dor'.
'dor'+'i'+'c'='DORIC'
(Other definitions for doric that I've seen before include "Type of classical architecture" , "A classical style of architecture with fluted columns" , "Plainest order of Gk. architecture" , "Classical building style" , "Greek architectural style" .)