Ridiculous bus turned in a road (6)
I believe the answer is:
absurd
'ridiculous' is the definition.
(synonyms)
'bus turned in a road' is the wordplay.
'turned' indicates an anagram.
'in' is an insertion indicator.
'road' becomes 'rd' (abbreviation for road).
'bus' with letters rearranged gives 'bsu'.
'a'+'rd'='ard'
'bsu' placed within 'ard' is 'ABSURD'.
(Other definitions for absurd that I've seen before include "'Ridiculous, silly (6)'" , "Incredible" , "Preposterous, nonsensical" , "Foolish, quite ridiculous" , "Arousing derision" .)