Definitely not name of nobleman left out (2,4)
I believe the answer is:
no fear
'definitely not' is the definition.
(I've seen this before)
'name of nobleman left out' is the wordplay.
I cannot really see how this works, but
'name' could be 'n' (common abbreviation e.g. on forms) and 'n' is present in the answer.
'of' could be 'o' and 'o' is present in the answer.
The remaining letters 'fear' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for no fear that I've seen before include "Informally, certainly not., who's afraid" , "Informally, not likely to do - who's afraid?" , "Definitely not (sl.)" , "Absolutely not" , "Far one (anag.)" .)