Um, a leftist was incorrect (5)
I believe the answer is:
erred
'was incorrect' is the definition.
(I've seen this before)
'um a leftist' is the wordplay.
'um' becomes 'er' (both said to fill in hesitation when speaking).
'a leftist' becomes 'red' (red can informally mean communist).
'er'+'red'='ERRED'
(Other definitions for erred that I've seen before include "slipped up" , "Made mistakes" , "Sinned" , "Made mistake, was incorrect" , "Went wrong" .)