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' says the letters should be written backwards.
'some of' says the answer is hidden in the clue.
'erus' can be found hidden inside 'the russians'.
'erus' backwards is '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" .)