Following line, I haven't managed to net a monster (9)
I believe the answer is:
leviathan
'monster' is the definition.
(I know that leviathan is a type of monster)
'following line i haven't managed to net a' is the wordplay.
'following' is a charade indicator (letters next to each other).
'line' becomes 'l' (used when specifying particular lines from a poem).
'managed' indicates anagramming the letters.
'to net' means one lot of letters goes inside another.
'l' after 'i' is 'il'.
'il'+'havent'='ilhavent'
'ilhavent' anagrammed gives 'leviathn'.
'leviathn' placed around 'a' is 'LEVIATHAN'.
(Other definitions for leviathan that I've seen before include "Huge (sea) monster" , "Biblical sea-monster" , "Monstrous sea creature" , "Monstrous beast" , "Gigantic" .)