In corrective use, it starts as it finishes (6)
I believe the answer is:
eraser
'it finishes' is the definition.
The definition suggests a singular noun which matches the answer.
'in corrective use it starts as' is the wordplay.
'in' indicates putting letters inside.
'corrective' becomes 'err' (I can't justify this - if you can you should believe this answer much more).
'use' is an anagram indicator.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'starts' says to put letters next to each other.
'e'+'as'='eas'
'eas' anagrammed gives 'ase'.
'err' going around 'ase' is 'ERASER'.
Can you help me to learn more?
(Other definitions for eraser that I've seen before include "Rubbing-out device" , "India rubber" , "Obliterator" , "Writing remover" , "Correcting implement used in drawing or writing" .)