Terribly valiant, he becomes a monster (9)
I believe the answer is:
leviathan
'a monster' is the definition.
(I know that leviathan is a type of mythical creature)
'terribly valiant he' is the wordplay.
'terribly' indicates an anagram.
'valiant'+'he'='valianthe'
'valianthe' is an anagram of 'LEVIATHAN'.
'becomes' acts as a link.
(Other definitions for leviathan that I've seen before include "Huge (sea) monster" , "An evil hat turns to anything of great size" , "Monstrous sea creature" , "Biblical sea-monster" , "Monstrous beast" .)