Going too far westward in small county seat (8)
I believe the answer is:
buttocks
'seat' is the definition.
(synonyms)
'going too far westward in small county' is the wordplay.
'going too far' becomes 'OTT' (short for over the top).
'westward' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'small county' becomes 'bucks' (short for Buckinghamshire; 'small' asks for an abbreviation).
'ott' reversed gives 'tto'.
'tto' inserted within 'bucks' is 'BUTTOCKS'.
(Other definitions for buttocks that I've seen before include "Bottom" , "Sit on them" , "Rear" , "Get behind" , "Body parts covered at least by 13" .)