Daft seaman knocked over American on road (6)
I believe the answer is:
absurd
'daft' is the definition.
(thesaurus)
'seaman knocked over american on road' is the wordplay.
'seaman' becomes 'ab'.
'knocked over' shows that the letters should be reversed in order.
'american' becomes 'us'.
'on' means one lot of letters go next to another.
'road' becomes 'rd' (abbreviation).
'us' reversed gives 'su'.
'ab'+'su'+'rd'='ABSURD'
(Other definitions for absurd that I've seen before include "Wildly illogical" , "Derisory, preposterous" , "word to describe Beckett's theatre!" , "Incredible" , "Idiotic" .)