Stop fellow about to shake crowded vehicle? (9)
I believe the answer is:
bandwagon
'crowded vehicle?' is the definition.
(as in jumping on a bandwagon)
'stop fellow about to shake' is the wordplay.
I cannot quite see how this works, but
'stop' could be 'go' (going is a kind of stopping) and 'go' is found in the answer.
'about' could be 'a' (abbreviation in Chambers) and 'a' is present in the answer.
This explanation may well be incorrect...
(Other definitions for bandwagon that I've seen before include "Astaire/Buchanan film" , "something fashionable all of a sudden" , "Vehicle (jumped on when suddenly popular?)" , "Fashionable movement" , "latest trend" .)