Egyptian goddess is appearing twice (4)
I believe the answer is:
'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' indicates the doubling of some letters.
'is' doubled is 'ISIS'.
'is' is the 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" .)