Enter reduction in error (9)
I believe the answer is:
introduce
'enter' is the definition.
('introduce' can be a synonym of 'enter')
'reduction in error' is the wordplay.
'in error' indicates an anagram.
'reduction' anagrammed gives 'INTRODUCE'.
(Other definitions for introduce that I've seen before include "Launch" , "Bring to notice or make known" , "Start; present" , "Make known to by way of reduction" , "Usher in" .)