At anvil, he forged a monster (9)
I believe the answer is:
leviathan
'a monster' is the definition.
(leviathan is a kind of monster)
'at anvil he forged' is the wordplay.
'forged' is an anagram indicator.
'at'+'anvil'+'he'='atanvilhe'
'atanvilhe' with letters rearranged gives 'LEVIATHAN'.
(Other definitions for leviathan that I've seen before include "Biblical sea monster" , "Autocratic monarch" , "Biblical sea-monster" , "Gigantic" , "Huge (sea) monster" .)