A certain number one in nor-nor-east (4)
I believe the answer is:
nine
'a certain number' is the definition.
'nine' can be an answer for 'number' (nine is a kind of number). I'm not sure about the 'a certain' bit.
'one in nor-nor-east' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'nor-nor-east' becomes 'nne' (I can't justify this - if you can you should give a lot more credence to this answer).
'i' put into 'nne' is 'NINE'.
Can you help me to learn more?
(Other definitions for nine that I've seen before include "A number of worthies" , "'Possession is... points of the law'" , "SECOND PART OF 14" , "A stitch in time saves them" , "and 23: Flash in the pan" .)