Max worried about one old adage (5)
I believe the answer is:
axiom
'adage' is the definition.
(I've seen this before)
'max worried about one old' is the wordplay.
'worried' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'old' becomes 'o' (abbreviation).
'max' with letters rearranged gives 'axm'.
'i'+'o'='io'
'axm' going around 'io' is 'AXIOM'.
(Other definitions for axiom that I've seen before include "Acceptable principle" , "Self-evident principle" , "Obviously true statement" , "A saying that's widely accepted as true" , "truthful statement" .)