A dour bent shown by American tough (7)
I believe the answer is:
arduous
'tough' is the definition.
(thesaurus)
'a dour bent shown by american' is the wordplay.
'bent' indicates anagramming the letters.
'shown by' says to put letters next to each other.
'american' becomes 'us'.
'dour' with letters rearranged gives 'rduo'.
'a'+'rduo'+'us'='ARDUOUS'
(Other definitions for arduous that I've seen before include "Tough" , "Fatiguing (task)" , "Demanding great effort" , "Difficult, laborious" , "Extremely taxing" .)