Blunt's backers regret being disloyal (6)
I believe the answer is:
untrue
'disloyal' is the definition.
(similar in meaning)
'blunt's backers regret' is the wordplay.
I cannot really see how this works, but
'regret' could be 'rue' (to rue something is to regret it) and 'rue' is found within the answer.
This explanation may well be incorrect...
'being' is the link.
Can you help me to learn more?
(Other definitions for untrue that I've seen before include "Sleeping around" , "Legendary" , "Spurious" , "disloyal" , "Not level" .)