Huge monster returning? No thanks, I've left! (9)
I believe the answer is:
leviathan
'huge monster' is the definition.
'leviathan' can be an answer for 'monster' (leviathan is a kind of monster). I am not certain of the 'huge' bit.
'returning? no thanks i've left' is the wordplay.
'returning?' is a reversal indicator.
'no' becomes 'nah' (I can't explain this - if you can you should believe this answer much more).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'left' becomes 'l' (common abbreviation).
'nah'+'ta'+'ive'+'l'='nahtaivel'
'nahtaivel' backwards is 'LEVIATHAN'.
Can you help me to learn more?
(Other definitions for leviathan that I've seen before include "Gigantic" , "Biblical sea monster" , "An evil hat turns to anything of great size" , "Monstrous beast" , "Biblical sea-monster" .)