Genuine as Russian leader? That's nonsense ultimately! (7)
I believe the answer is:
sincere
'genuine' is the definition.
('sincere' can be a synonym of 'genuine')
'as russian leader? that's nonsense ultimately' is the wordplay.
'as' becomes 'since' (I've seen this before**).
'leader?' says to take the initial letters (I've seen 'leader' mean this).
'that's' says to put letters next to each other.
'ultimately' indicates one should take the final letters.
The initial letter of 'russian' is 'r'.
The final letter of 'nonsense' is 'e'.
'since'+'r'+'e'='SINCERE'
(Other definitions for sincere that I've seen before include "Free from deceit" , "Unfeigned, genuine" , "Showing genuine feelings" , "Earnest" , "Open and genuine, not deceitful" .)