Artist's picture heartily upset Russian leader (4)
I believe the answer is:
tsar
'russian leader' is the definition.
(I've seen this before)
'artist's picture heartily upset' is the wordplay.
'artist's' becomes 'ras' (Royal Academician, member of Royal Academy of Arts).
'heartily' indicates the central letters (the inside or heart of the word).
'upset' says the letters should be written backwards.
The central letter of 'picture' is 't'.
'ras'+'t'='rast'
'rast' backwards is 'TSAR'.
(Other definitions for tsar that I've seen before include "Once-powerful Russian" , "specific authority" , "Russian leader" , "Tyrant" , "Peter the Great, for example" .)