Note one severe region (8)
I believe the answer is:
district
'region' is the definition.
(district is a kind of region)
'note one severe' is the wordplay.
'note' becomes 'D' (musical note).
'one' becomes 'i' (Roman numeral).
'severe' becomes 'strict' (I've seen this in another clue).
'd'+'i'+'strict'='DISTRICT'
(Other definitions for district that I've seen before include "Area, region" , "Territory, zone" , "'Area, territory (8)'" , "Zone, area" , "Region marked off for administrative or other purposes" .)