He, valiant, destroyed monster (9)
I believe the answer is:
leviathan
'monster' is the definition.
(leviathan is a kind of monster)
'he valiant destroyed' is the wordplay.
'destroyed' indicates anagramming the letters.
'he'+'valiant'='hevaliant'
'hevaliant' anagrammed gives 'LEVIATHAN'.
(Other definitions for leviathan that I've seen before include "Monstrous sea creature" , "Biblical sea monster" , "Monstrous beast" , "An evil hat turns to anything of great size" , "Gigantic" .)