Saying nothing in a mix up (5)
I believe the answer is:
axiom
'saying' is the definition.
(axiom is a kind of saying)
'nothing in a mix up' is the wordplay.
'nothing in' becomes 'o' (I've seen this in other clues).
'up' indicates an anagram.
'o'+'a'+'mix'='oamix'
'oamix' anagrammed gives 'AXIOM'.
(Other definitions for axiom that I've seen before include "Basic proposition" , "Obviously true statement" , "fundamental principle" , "Universal principle" , "Truism - proverb" .)