Wise guy immediately into gear, leading endless car race (4-2-3)
I believe the answer is:
know-it-all
'wise guy' is the definition.
(thesaurus)
'immediately into gear leading endless car race' is the wordplay.
'immediately' becomes 'now' (I've seen this before).
'into' means one lot of letters goes inside another.
'gear' becomes 'kit' (kit is a kind of gear).
'leading endless car race' becomes 'all' (I can't justify this - if you can you should give a lot more credence to this answer).
'now' going into 'kit' is 'knowit'.
'knowit'+'all'='KNOW-IT-ALL'
Can you help me to learn more?
(Other definitions for know-it-all that I've seen before include "Unteachable bigot" , "Smart alec" .)