Royal Navy brought in to subdue everyone in Duchy (8)
I believe the answer is:
cornwall
'duchy' is the definition.
(territory of the Duke of Cornwall)
'royal navy brought in to subdue everyone' is the wordplay.
'royal navy' becomes 'RN' (abbreviation for Royal Navy).
'brought in' indicates putting letters inside.
'to subdue' becomes 'cow' (as in cowing someone into submission).
'everyone' becomes 'all' (all people).
'rn' put into 'cow' is 'cornw'.
'cornw'+'all'='CORNWALL'
'in' acts as a link.
(Other definitions for cornwall that I've seen before include "Land" , "County of south-west England" , "Southwestern English county" , "Crown all in the English county, strangely" , "County touching only one other" .)