Political allies upset about party? That's a puzzle (6)
I believe the answer is:
sudoku
'that's a puzzle' is the definition.
(I've seen this before)
'political allies upset about party?' is the wordplay.
'political allies' becomes 'UK US' (United Kingdom and United States).
'upset' is a reversal indicator.
'about' is an insertion indicator.
'party?' becomes 'do' (do is a kind of party).
'ukus' reversed gives 'suku'.
'suku' going around 'do' is 'SUDOKU'.
(Other definitions for sudoku that I've seen before include "Puzzle involving the numbers 1 to 9" , "Number placement puzzle with a 9 x 9 grid" , "(Usually) nine-by-nine numbers puzzle" , "Popular logic-based puzzle" , "numbers in boxes" .)