A bus broken down over the road? How ridiculous! (6)
I believe the answer is:
absurd
'ridiculous' is the definition.
(synonyms)
'a bus broken down over the road?' is the wordplay.
'broken down' is an anagram indicator.
'over' is an insertion indicator.
'the road?' becomes 'rd' (abbreviation).
'bus' is an anagram of 'bsu'.
'a' inserted into 'bsu' is 'absu'.
'absu'+'rd'='ABSURD'
'how' acts as a link.
(Other definitions for absurd that I've seen before include "Arousing derision" , "Highly illogical" , "Quite ridiculous, silly" , "10 18's [SAMUAL BECKETT] style of theatre" , "Ridiculous, illogical" .)