Number over eight, initially (3)
I believe the answer is:
one
'number' is the definition.
(I know that one is a type of digit)
'over eight initially' is the wordplay.
'over' becomes 'on' (physically on top of).
'initially' indicates taking the first letters.
The initial letter of 'eight' is 'e'.
'on'+'e'='ONE'
(Other definitions for one that I've seen before include "Single person or thing" , "1" , "Identical; number" , "This is singular" , "Single, sole" .)