Maxim about mishap involving gold (8)
I believe the answer is:
aphorism
'maxim' is the definition.
(I know that aphorism is a type of maxim)
'about mishap involving gold' is the wordplay.
'about' means one lot of letters goes inside another.
'involving' indicates an anagram.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'mishap' is an anagram of 'aphism'.
'aphism' placed around 'or' is 'APHORISM'.
(Other definitions for aphorism that I've seen before include "Brief pithy saying" , "Maxim, short proverb" , "Short, clever saying" , "Saw" , "Adage" .)