Sanity - there's possibly nothing saner (6)
I believe the answer is:
reason
'sanity' is the definition.
(reason is a kind of sanity)
'possibly nothing saner' is the wordplay.
'possibly' is an anagram indicator.
'nothing' becomes 'O' (O resembles 0).
'o'+'saner'='osaner'
'osaner' with letters rearranged gives 'REASON'.
'there's' acts as a link.
(Other definitions for reason that I've seen before include "logical argument" , "Sanity, the ability to think" , "Employ logic" , "Justification (for action)" , "Motivation or cause" .)