One Liberal assuming instinctive 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')
'one liberal assuming instinctive' is the wordplay.
'one' becomes 'i' (roman numeral).
'liberal' becomes 'l' (abbreviation).
'assuming' is an insertion indicator (assume can mean to take on).
'instinctive' becomes 'gut' (I've seen this in other clues).
'i'+'l'='il'
'il' put into 'gut' is 'GUILT'.
(Other definitions for guilt that I've seen before include "(Feeling induced by) wrongdoing" , "uncomfortable feeling" , "(Shame from) having done wrong" , "Remorse caused by feelings of responsibility" , "Bad conscience" .)