Partially backing Starmer as the leader (4)
I believe the answer is:
tsar
'leader' is the definition.
(type of leader)
'partially backing starmer as the' is the wordplay.
'backing' says the letters should be written in reverse.
'partially' says the answer is hidden in the clue.
'rast' can be found hidden inside 'starmer as the'.
'rast' reversed gives 'TSAR'.
(Other definitions for tsar that I've seen before include "One associated with 26 [KREMLIN]" , "Nicholas II, say" , "Supreme authority" , "Supremo" , "A former Russian ruler - rats to him" .)