Wide county, an ancient English territory (6)
I believe the answer is:
wessex
'an ancient english territory' is the definition.
The definition suggests a singular noun which matches the answer.
'wide county' is the wordplay.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'county' becomes 'essex' (English county).
'w'+'essex'='WESSEX'
Can you help me to learn more?
(Other definitions for wessex that I've seen before include "Anglo-Saxon kingdom" , "Of which Alfred the Great was king" , "Hardy's West Country" , "Thomas Hardy country" , "Thomas Hardy's Dorset" .)