Instinctive to accept one criminal's ultimate responsibility (5)
I believe the answer is:
guilt
'responsibility' is the definition.
(I have seen 'Criminal responsibility ' mean 'guilt' so perhaps 'responsibility' could also mean 'guilt')
'instinctive to accept one criminal's ultimate' is the wordplay.
'instinctive' becomes 'gut' (I've seen this in other clues).
'to accept' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'ultimate' says to take the final letters.
The last letter of 'criminal' is 'l'.
'i'+'l'='il'
'gut' going around 'il' is 'GUILT'.
(Other definitions for guilt that I've seen before include "Remorse, feeling responsible for an offence or action" , "Sin" , "State of having committed offence" , "Remorse caused by feelings of responsibility" , "Consciousness of one's wrongdoing" .)