One putting on left-wing prosecutor's rebuffed (5)
I believe the answer is:
adder
'one putting on' is the definition.
(someone adding or putting things on)
'left-wing prosecutor's rebuffed' is the wordplay.
'left wing' becomes 'red' (red can informally mean communist).
'prosecutor' becomes 'D.A.' (District Attorney).
'rebuffed' says the letters should be written backwards.
'red'+'da'='redda'
'redda' reversed gives 'ADDER'.
(Other definitions for adder that I've seen before include "Small poisonous snake, viper" , "Totting snake" , "Dread (anag)" , "Type of snake" , "One contributing more" .)