State area in central Monmouthshire (4)
I believe the answer is:
utah
'state' is the definition.
(Utah is an example)
'area in central monmouthshire' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'in' indicates putting letters inside.
'central' indicates the central letters.
The central letters of 'monmouthshire' are 'uth'.
'a' inserted into 'uth' is 'UTAH'.
(Other definitions for utah that I've seen before include "The "Beehive" State, capital Salt Lake City" , "American location" , "Mormon US state" , "US state settled by the Mormons" , "The American Mormon state" .)