Peter for one ruled heartlessly in retrospect (6)
I believe the answer is:
denier
'peter' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'one ruled heartlessly in retrospect' is the wordplay.
'one ruled' becomes 'reigned' (I've seen this in another clue).
'heartlessly' suggests removing the centre.
'in retrospect' says the letters should be written backwards (in retrospect means looking back).
'reigned' with its middle taken out is 'reined'.
'reined' written backwards gives 'DENIER'.
'for' is the link.
Can you help me to learn more?
(Other definitions for denier that I've seen before include "Conspiracist probably" , "Units of weight in silks, nylon and other yarns" , "Person refusing to admit (something)" , "Unit of weight for fine material, silk or nylon" , "measure of fine quality" .)