Dull-witted American president's thrown outside (10)
I believe the answer is:
pedestrian
'dull-witted' is the definition.
(pedestrian can mean dull or boring)
'american president's thrown outside' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'thrown' is an anagram indicator.
'outside' means one lot of letters goes inside another.
'president' with letters rearranged gives 'pedestrin'.
'a' placed inside 'pedestrin' is 'PEDESTRIAN'.
(Other definitions for pedestrian that I've seen before include "chap on zebra could be?" , "lame" , "he goes for a walk" , "wearer of leather?" , "Dull; walker" .)