Rotten sarnie -- ignoring one's more sensible (5)
I believe the answer is:
saner
'more sensible' is the definition.
'rotten sarnie ignoring one's' is the wordplay.
'rotten' indicates anagramming the letters.
'ignoring' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'sarnie' with 'i' removed is 'sarne'.
'sarne' is an anagram of 'SANER'.
(Other definitions for saner that I've seen before include "Less mad" , "less as hatter?" .)