Further, nothing true is false (5)
I believe the answer is:
outer
'further' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I don't see?
'nothing true is false' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'is false' is an anagram indicator.
'true' with letters rearranged gives 'uter'.
'o'+'uter'='OUTER'
Can you help me to learn more?
(Other definitions for outer that I've seen before include "Outside; external" , "Archer's shot" , "Away from the centre" , "Exterior or external, like space" , "Route (anag.)" .)