One who modifies mare is wrong about tail (7)
I believe the answer is:
amender
'one who modifies' is the definition.
The definition suggests a singular noun which matches the answer.
'mare is wrong about tail' is the wordplay.
'is wrong' is an anagram indicator.
'about' is an insertion indicator.
'tail' becomes 'end' (the tail of something can mean the end).
'mare' anagrammed gives 'amer'.
'amer' enclosing 'end' is 'AMENDER'.
Can you help me to learn more?
(Other definitions for amender that I've seen before include "agent of change" , "Editor" .)