100 in old county division (7)
I believe the answer is:
hundred
This clue is a double definition.
'100' is the first definition.
(synonyms)
'old county division' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for hundred that I've seen before include "Former division of county" , "number of scores" , "English county division" , "so many others" , "Number; county subdivision" .)