Socialist, after hesitation, made a mistake (5)
I believe the answer is:
erred
'made a mistake' is the definition.
('err' can be a synonym of 'make a mistake')
'socialist after hesitation' is the wordplay.
'socialist' becomes 'red' (red can informally mean a socialist).
'after' says to put letters next to each other.
'hesitation' becomes 'er' (sound made while hesitating - um, er).
'red' after 'er' is 'ERRED'.
(Other definitions for erred that I've seen before include "Gone astray!" , "Sinned" , "Went astray" , "Committed a sin" , "Went wrong" .)