Number going into eighteen I notched up (4)
I believe the answer is:
nine
'number going' is the definition.
'nine' can be an answer for 'number' (I've seen this before). I am not sure about the 'going' bit.
'into eighteen i notched up' is the wordplay.
'into' says the answer is hidden in the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'enin' can be found hidden inside 'eighteen i notched'.
'enin' reversed gives 'NINE'.
Can you help me to learn more?
(Other definitions for nine that I've seen before include "and 23: Flash in the pan" , "'Possession is... points of the law'" , "3x3" , "Number of feline existences" , "A number of worthies" .)