She's backward in reading (3)
I believe the answer is:
ida
'she's' is the definition.
(I've seen this before)
'backward in reading' is the wordplay.
'backward' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'adi' can be found hidden inside 'reading'.
'adi' written backwards gives 'IDA'.
(Other definitions for ida that I've seen before include "Greek mount - Gilbertian princess" , "Princess __ (G&S comic opera)" , "Cretan mountain - Gilbert and Sullivan princess" , "Name of girl or mountain" , "Poetic princess" .)