Squeeze beneath a horse: it makes gelding niggled (7)

I believe the answer is:
anagram
'it makes gelding niggled' is the definition.
The definition suggests a singular noun which matches the answer.
'squeeze beneath a horse' is the wordplay.
'squeeze' becomes 'ram' (I can't explain this - if you can you should give a lot more credence to this answer).
'beneath' says to put letters next to each other (in a down clue).
'horse' becomes 'nag' (term for an old horse).
'a'+'nag'='anag'
'ram' after 'anag' is 'ANAGRAM'.
Can you help me to learn more?
(Other definitions for anagram that I've seen before include "Gelding can be niggled by this" , "characters on the move" , ""Diagnose" is one of "San Diego"" , "One way to clue" , "Type of word puzzle" .)
