Reduce tension in defeat: a tyrant's leadership rejected across America (6)
I believe the answer is:
defuse
'reduce tension' is the definition.
(as in defusing a tense situation)
'defeat a tyrant's leadership rejected across america' is the wordplay.
I cannot quite see how this works, but
'america' could be 'us' (abbreviation for the United States) and 'us' is located in the answer.
This may be the basis of clue (or it may be nonsense).
'in' is the link.
(Other definitions for defuse that I've seen before include "Make (bomb) safe" , "Remove risk of explosion" , "undertake bomb disposal" , "Reduce dangerous tension or make explosive safe" , "Remove the triggering device" .)