Adieu loyal sudoku solver: you ultimately rock (5)
I believe the answer is:
uluru
'rock' is the definition.
(New Zealand landmark also known as Ayers Rock)
'adieu loyal sudoku solver you ultimately' is the wordplay.
'ultimately' indicates one should take the final letters.
The last letters of 'adieu loyal sudoku solver you' are 'ULURU'.
(Other definitions for uluru that I've seen before include "Australian landmark" , "Sacred Australian rock" , "Aussie rock" , "Aboriginal native name for Ayers Rock" .)