Particular group bridging river in Southwestern county (8)
I believe the answer is:
somerset
'county' is the definition.
(English county)
'particular group' is the wordplay.
'particular' becomes 'somer' (I can't justify this - if you can you should believe this answer much more).
'group' becomes 'set' (both can mean a collection).
'somer'+'set'='SOMERSET'
'bridging river in southwestern' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for somerset that I've seen before include "SW county" , "English territory" , "western county" , "Cheddar Gorge's county" , "Rome's set up in this English county" .)