UK duo's playing the numbers game? (6)
I believe the answer is:
sudoku
'numbers game?' is the definition.
(I've seen this in another clue)
'uk duo's playing' is the wordplay.
'playing' indicates anagramming the letters.
'uk'+'duos'='ukduos'
'ukduos' is an anagram of 'SUDOKU'.
'the' acts as a link.
(Other definitions for sudoku that I've seen before include "filling boxes" , "Puzzle involving the numbers 1 to 9" , "9 x 9, 1-9 puzzle" , "Number placement puzzle with a 9 x 9 grid" , "Popular logic-based puzzle" .)