A “B road” bearing us westwards is ludicrous (6)
I believe the answer is:
absurd
'is ludicrous' is the definition.
(synonyms)
'a b road bearing us westwards' is the wordplay.
'road' becomes 'rd' (abbreviation).
'bearing' means one lot of letters go next to another.
'westwards' is a reversal indicator (letters go right to left or from 'east' to 'west').
'us' in reverse letter order is 'su'.
'rd' put after 'su' is 'surd'.
'a'+'b'+'surd'='ABSURD'
(Other definitions for absurd that I've seen before include "irrational" , "Arousing derision" , "Highly illogical" , "Derisory, preposterous" , "Ridiculous, illogical" .)