Do without the first number that's overlooked (9)
I believe the answer is:
forgotten
'overlooked' is the definition.
(I've seen this in another clue)
'do without the first number' is the wordplay.
'do without' becomes 'forgo' (I've seen this before).
'the first' becomes 't' (1st letter of 'the').
'number' becomes 'ten' (ten is a kind of number).
'forgo'+'t'+'ten'='FORGOTTEN'
'that's' acts as a link.
(Other definitions for forgotten that I've seen before include "Ignored" , "Disregarded" , "Neglected" , "Overlooked" , "Gone from memory" .)