One's driven into a bottleneck in county town, with crowd hemming in Sierra (9)
I believe the answer is:
corkscrew
'one's driven into a bottleneck in county' is the definition.
I can't judge whether this defines the answer.
'town with crowd hemming in sierra' is the wordplay.
'town with' becomes 'cork' (I can't justify this - if you can you should give a lot more credence to this answer).
'crowd' becomes 'crew' (synonyms).
'hemming in' means one lot of letters goes inside another (inserted letters are hemmed in).
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'cork'+'crew'='corkcrew'
'corkcrew' placed around 's' is 'CORKSCREW'.
Can you help me to learn more?
(Other definitions for corkscrew that I've seen before include "Move in a spiral manner" , "Bordeaux might require this?" , "Graves opened by this" , "handy opener" , "This might help one who gets in the drink" .)