Saw old king in horrible mishap (8)
I believe the answer is:
aphorism
'saw' is the definition.
(saw can mean a proverb or saying)
'old king in horrible mishap' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'in' indicates putting letters inside.
'horrible' indicates anagramming the letters.
'o'+'r'='or'
'mishap' is an anagram of 'aphism'.
'or' placed into 'aphism' is 'APHORISM'.
(Other definitions for aphorism that I've seen before include "Terse saying" , "One could briefly say this" , "pithy statement" , "Short, clever saying" , "Brief pithy saying" .)