Valiant, he wrestles sea-monster (9)
I believe the answer is:
leviathan
'sea-monster' is the definition.
(I've seen this before)
'valiant he wrestles' is the wordplay.
'wrestles' is an anagram indicator (I've seen 'wrestle' mean this (I've seen this in other clues)).
'valiant'+'he'='valianthe'
'valianthe' with letters rearranged gives 'LEVIATHAN'.
(Other definitions for leviathan that I've seen before include "An evil hat turns to anything of great size" , "Biblical sea monster" , "Monstrous sea creature" , "Gigantic" , "Monstrous beast" .)