Old governer modelled a gentle form of punishment (6)
I believe the answer is:
satrap
'old governer modelled a gentle' is the definition.
I can't tell whether this defines the answer.
'form of punishment' is the wordplay.
'form of' becomes 'sat' (I can't explain this - if you can you should give a lot more credence to this answer).
'punishment' becomes 'rap' (as in taking the rap for something).
'sat'+'rap'='SATRAP'
Can you help me to learn more?
(Other definitions for satrap that I've seen before include "Old Persian ruler" , "A sprat for the old governor of Persia" , "local ruler" , "petty tyrant" , "Old Persian governor - a sprat (anag)" .)