Answer I did not change in simple maths (8)
I believe the answer is:
addition
'simple maths' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'answer i did not change' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'change' indicates anagramming the letters.
'i'+'did'+'not'='ididnot'
'ididnot' anagrammed gives 'ddition'.
'a'+'ddition'='ADDITION'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for addition that I've seen before include "I don't aid (anag.)" , "Supplement" , "Combination of numbers to find the sum" , "Reckoning" , "an arithmetical process" .)