It's right to regret coming to the worst conclusion (4)
I believe the answer is:
true
'it's right' is the definition.
(associated in meaning)
'regret coming to the worst conclusion' is the wordplay.
'regret' becomes 'rue' (synonyms).
'coming to' means one lot of letters go next to another.
'conclusion' suggests the final letters.
The last letter of 'worst' is 't'.
'rue' put after 't' is 'TRUE'.
'to' acts as a link.
(Other definitions for true that I've seen before include "Unwavering" , "Genuine, real" , "Real; loyal" , "Unfeigned" , "Right - honourable" .)