I'm back again, girl! (4)
I believe the answer is:
mimi
'girl' is the definition.
'i'm back again' is the wordplay.
'back' is a reversal indicator.
'again' becomes 'mi' (I can't justify this - if you can you should give a lot more credence to this answer).
'im' reversed gives 'mi'.
'mi'+'mi'='MIMI'
Can you help me to learn more?
(Other definitions for mimi that I've seen before include "Puccini heroine" , "I'm, I'm mad about the heroine in 'La Boheme'" , "showgirl" , "Bohemian girl" , "Puccini's seamstress" .)