Gelding niggled by this horse a sheep defends (7)
I believe the answer is:
anagram
'gelding niggled by this' is the definition.
The definition suggests a singular noun which matches the answer.
'horse a sheep defends' is the wordplay.
'horse' becomes 'nag' (nag is a kind of horse).
'sheep' becomes 'ram' (ram is a kind of sheep).
'defends' indicates putting letters inside.
'a'+'ram'='aram'
'nag' inserted inside 'aram' is 'ANAGRAM'.
Can you help me to learn more?
(Other definitions for anagram that I've seen before include "There's confusion among characters" , "Word with rearrangeable letters" , "5 down is one" , "Word made by rearranging the letters of another" , "Word(s) rearranged to form others" .)