Roman with one supporter defending county (7)
I believe the answer is:
cumbria
'county' is the definition.
(I know that Cumbria is a county)
'roman with one supporter defending' is the wordplay.
'roman' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'with' becomes 'cum' ('with' in Latin).
'one supporter' becomes 'bra' (a bra gives support to breasts).
'defending' is an insertion indicator.
'cum'+'bra'='cumbra'
'i' going within 'cumbra' is 'CUMBRIA'.
Can you help me to learn more?
(Other definitions for cumbria that I've seen before include "Northern county" , "Northwest English county" , "Keswick's county" , "County of NW England" , "County of northwest England" .)