Top-selling Instagram mum is a symbol in Europe (5)
I believe the answer is:
sigma
'europe' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'top-selling instagram mum is a symbol in' is the wordplay.
'top' suggests taking the first letters.
'mum' becomes 'ma' (ma can mean mother).
'is a symbol' becomes 'g' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
The initial letters of 'selling instagram' is 'si'.
'si'+'ma'='sima'
'sima' going around 'g' is 'SIGMA'.
Can you help me to learn more?
(Other definitions for sigma that I've seen before include "The 18th letter of the Greek alphabet" , "Foreign character" , "character in Athens" , "letter to Greeks" , "Greek letter indicating summation in maths" .)