False regret after relative's guillotined (6)
I believe the answer is:
untrue
'false' is the definition.
(I know that false can be written as untrue)
'regret after relative's guillotined' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'after' means one lot of letters go next to another.
'relative' becomes 'aunt' (aunt is a kind of relative).
'guillotined' means to remove the first letter.
'aunt' with its initial letter taken off is 'unt'.
'rue' put after 'unt' is 'UNTRUE'.
(Other definitions for untrue that I've seen before include "Fallacious" , "disloyal" , "Not correct, like a lie" , "Counter-factual" , "not right!" .)