The phrase "no fear" means,Definitely,will do,don't you worry about that mate(Australian slang?? maybe).
"Definitely Not" I can't really explain this properly but I think i have it right(in my mind, at least).
No fear - can mean both.
Definitely and Definitely Not (of course I won't)
and I think it could be tied to that American sarcastic "not"
eg. Omar, I love you ...snigger..tee-he.....NOT.
am I making any sense?