Egyptian goddess with one little sister (4)
I believe the answer is:
isis
'egyptian goddess' is the definition.
(I've seen this in another clue)
'with one little sister' is the wordplay.
'with' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'little sister' becomes 'sis' (synonyms. I am not sure about the 'little' bit.).
'i'+'sis' is 'ISIS'.
Can you help me to learn more?
(Other definitions for isis that I've seen before include "Oxford's second eight" , "where one might take a punt" , "Si, si - an Egyptian goddess" , "Egyptian goddess and jihadist group" , "God" .)