Montgomery is in this year overwhelmed by war captives (5)
I believe the answer is:
powys
'montgomery is in this year' is the definition.
The definition suggests a singular noun which matches the answer.
'overwhelmed by war captives' is the wordplay.
'overwhelmed by' indicates putting letters inside (inserted letters get absorbed or overwhelmed).
'war' becomes 'y' (I can't justify this - if you can you should believe this answer much more).
'captives' becomes 'POWs' (prisoners of war).
'y' inserted into 'pows' is 'POWYS'.
Can you help me to learn more?
(Other definitions for powys that I've seen before include "area of Wales" , "John Cowper -, twentieth century Welsh novelist" , "Largest Welsh county" , "He wrote A Glastonbury Romance" , "County in Mid Wales" .)