Force introduced to subdue entire Celtic region (8)
'celtic region' is the definition.
(region of England)
'force introduced to subdue entire' is the wordplay.
'force' becomes 'rn' (abbreviation for Royal Navy).
'introduced to' is an insertion indicator.
'subdue' becomes 'cow' (I've seen this before).
'entire' becomes 'all' (I've seen this before).
'rn' put into 'cow' is 'cornw'.
(Other definitions for cornwall that I've seen before include "Land", "County touching only one other", "Duchy", "County of south-west England", "Southern English county".)