County allied with rose, briefly (7)
I believe the answer is:
cumbria
'county' is the definition.
(I've seen this before)
'allied with rose briefly' is the wordplay.
'allied with' becomes 'cum' (the Latin word 'cum' can mean 'combined with').
'rose' becomes 'briar' (briar is a kind of rose).
'briefly' means to remove the last letter.
'briar' with its last letter taken off is 'bria'.
'cum'+'bria'='CUMBRIA'
(Other definitions for cumbria that I've seen before include "Keswick's county" , "County of northwest England" , "County of NW England" , "Lake district area" , "Northwest English county" .)