Hero fought at clashes, as planned in advance (12)
I believe the answer is:
aforethought
'planned in advance' is the definition.
(I've seen this before)
'hero fought at clashes' is the wordplay.
'clashes' indicates anagramming the letters (I've seen 'clashing' mean this (stop)).
'hero'+'fought'+'at'='herofoughtat'
'herofoughtat' is an anagram of 'AFORETHOUGHT'.
'as' acts as a link.
(Other definitions for aforethought that I've seen before include "Premeditated" , "planned ahead" , "Conceived in advance" , "planned in advance" .)