The terrible fellow twice turned traitor! (6)
I believe the answer is:
tartar
'the terrible fellow' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'twice turned traitor' is the wordplay.
'twice' shows that letters should be duplicated.
'turned' says the letters should be written in reverse.
'traitor' becomes 'rat' (rat can mean a treacherous person).
'rat' written backwards gives 'tar'.
'tar' doubled is 'TARTAR'.
Can you help me to learn more?
(Other definitions for tartar that I've seen before include "Deposit that forms on teeth" , "Hard, crusty deposit on the teeth" , "Crusty deposits on teeth" , "Crust on teeth or wine deposit" , "plaque" .)