One of eight? (7)
I believe the answer is:
integer
I believe this is a double definition.
'one' is the first definition.
(one is an integer)
'eight?' is the second definition.
(eight is a kind of integer)
'of' acts as a link.
(Other definitions for integer that I've seen before include "Gin tree (anag)" , "One for example" , "Whole unit" , "The whole thing" , "one or two, perhaps" .)