United States under old tyrant is oppressive (7)
I believe the answer is:
onerous
'oppressive' is the definition.
(I've seen this in another clue)
'united states under old tyrant' is the wordplay.
'united states' becomes 'US' (abbreviation for the United States).
'under' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tyrant' becomes 'nero' (Nero was a Roman tyrant).
'o'+'nero'='onero'
'us' put after 'onero' is 'ONEROUS'.
'is' is the link.
(Other definitions for onerous that I've seen before include "Burdensome, taxing" , "Involving a great deal of effort" , "Difficult, troublesome" , "Burdensome, oppressive" , "Effortful" .)