Egyptian goddess is appearing twice (4)
I believe the answer is:
isis
'egyptian goddess' is the definition.
(I've seen this in another clue)
'appearing twice' is the wordplay.
'appearing' becomes 'is' (I can't justify this - if you can you should give a lot more credence to this answer).
'twice' shows that letters should be duplicated.
'is' duplicated is 'ISIS'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for isis that I've seen before include "River Thames at Oxford" , "Wife of Osiris" , "Oxfordshire river" , "To be, to be an Egyptian goodess" , "Mrs Osiris" .)