Highly illogical (6)
I believe the answer is:
absurd
'highly illogical' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for absurd that I've seen before include "'Derisory, cockeyed (6)'" , "Quite ridiculous, silly" , "Highly illogical" , "Patently ridiculous" , "Derisory, preposterous" .)