Ill-treated the American in a plot (6)
I believe the answer is:
abused
'ill-treated' is the definition.
('abuse' can be a synonym of 'ill-treat')
'american in a plot' is the wordplay.
'american' becomes 'US' (abbreviation for United States).
'in' indicates putting letters inside.
'plot' becomes 'bed' (a plot of land).
'a'+'bed'='abed'
'us' inserted inside 'abed' is 'ABUSED'.
'the' is the link.
(Other definitions for abused that I've seen before include "Wrongfully treated" , "Took undue advantage of" , "Swore at" , "Reviled, called nasty names" , "Reviled, aimed verbal assault at" .)