Certain to upset some of the Russians (4)
I believe the answer is:
sure
'certain' is the definition.
(similar in meaning)
'upset some of the russians' is the wordplay.
'upset' shows that the letters should be reversed in order.
'some of' indicates a hidden word.
'erus' is hidden within 'the russians'.
'erus' written backwards gives 'SURE'.
'to' is the link.
(Other definitions for sure that I've seen before include "Certain to be a ruse" , "Agreed" , "Safe" , "Feeling no doubt" , "certainly in the USA" .)