Nut chewed with regret? It's like a pork pie (6)
I believe the answer is:
untrue
'like a pork pie' is the definition.
('pork pie' is Cockney rhyming slang for 'lie')
'nut chewed with regret?' is the wordplay.
'chewed' is an anagram indicator.
'with' means one lot of letters go next to another.
'regret?' becomes 'rue' ('rue' can be a synonym of 'regret').
'nut' is an anagram of 'unt'.
'unt'+'rue'='UNTRUE'
'it's' is the link.
(Other definitions for untrue that I've seen before include "Not in accordance with fact" , "Not loyal" , "Not faithful" , "Not level" , "Counter-factual" .)