Write numbers in a chain (8)
I believe the answer is:
pennines
'a chain' is the definition.
(I have seen 'chain of hills' mean 'pennines' so perhaps 'chain' could also mean 'pennines')
'write numbers' is the wordplay.
'write' becomes 'pen' ('pen' can be a synonym of 'write').
'numbers' becomes 'nines' (nine is a kind of number**).
'pen'+'nines'='PENNINES'
'in' is the link.
(Other definitions for pennines that I've seen before include "English mountain range" , "Range of hills in England" , "chain of hills" , "Backbone of England" .)