Ludicrous unlimited cabs US rejected on highway (6)
I believe the answer is:
absurd
'ludicrous' is the definition.
(synonyms)
'unlimited cabs us rejected on highway' is the wordplay.
'unlimited' says to take the centre (discard the edges or 'limits' of the word).
'rejected' says the letters should be written in reverse.
'on' means one lot of letters go next to another.
'highway' becomes 'rd'.
The middle letters of 'cabs' are 'ab'.
'us' reversed gives 'su'.
'ab'+'su'+'rd'='ABSURD'
(Other definitions for absurd that I've seen before include "Wildly illogical" , "Patently ridiculous" , "Really ridiculous, silly" , "Ridiculous, ludicrous" , "'Derisory, cockeyed (6)'" .)