You’re right in the first instance to have regrets (4)
I believe the answer is:
true
'you're right' is the definition.
(I've seen this in another clue)
'in the first instance to have regrets' is the wordplay.
'in the first instance' suggests taking the first letters.
'have regrets' becomes 'rue' (I've seen this in another clue).
The initial letter of 'to' is 't'.
't'+'rue'='TRUE'
(Other definitions for true that I've seen before include "Right; genuine" , "Accurate, factual" , "Accurate, exact" , "Supported by the facts" , "Corrected" .)