Twice she tells you she's coming back (4)
I believe the answer is:
mimi
'tells you she's coming back' is the definition.
I can't tell whether this definition defines the answer.
'twice she' is the wordplay.
'twice' indicates the doubling of some letters.
'she' becomes 'mi' (I can't explain this - if you can you should believe this answer much more).
'mi' doubled is 'MIMI'.
Can you help me to learn more?
(Other definitions for mimi that I've seen before include "Starving character of "La Boheme"" , "showgirl" , "La Boheme character (whose tiny hand is frozen)" , "I'm upset twice by the operatic girl" , "I'm, I'm mad about the heroine in 'La Boheme'" .)