County town base for duke (7)
I believe the answer is:
bedford
'county town' is the definition.
(county town of Bedfordshire)
'base for duke' is the wordplay.
'base' becomes 'bed' (bed is a kind of base).
'duke' becomes 'd' (abbreviation).
'bed'+'for'+'d'='BEDFORD'
(Other definitions for bedford that I've seen before include "English county town" , "County town to north of London" .)