Definitely not! (2,4)
I believe the answer is:
no fear
'definitely not' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for no fear that I've seen before include "what the bold show" , "What 7 [INTREPID] person will have" , "Definitely not (informal)" , "Far one (anag.)" , "What brave man displays" .)