Russian leader succeeded in turning informer (4)
I believe the answer is:
tsar
'russian leader' is the definition.
(I've seen this before)
'succeeded in turning informer' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'in' is an insertion indicator.
'turning' is a reversal indicator.
'informer' becomes 'rat' ('rat' can be a synonym of 'informer').
'rat' reversed gives 'tar'.
's' inserted into 'tar' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Powerful man" , "A former Russian ruler - rats to him" , "Russian leader" , "Supreme authority" , "Peter the Great, for example" .)