Second most important road sign (4)
I believe the answer is:
stop
'sign' is the definition.
(stop is a kind of sign)
'second most important road' is the wordplay.
'second' becomes 's' (common abbreviation).
'most important road' becomes 'top' (I am not sure about this - if you are sure you should believe this answer much more).
's'+'top'='STOP'
Can you help me to learn more?
(Other definitions for stop that I've seen before include "Plug; cease" , "period in US" , "Intercept" , "This watch is used to time races" , "Put an end to" .)