She's back in ballet (4)
I believe the answer is:
ella
'she's' is the definition.
(I've seen this before)
'back in ballet' is the wordplay.
'back' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'alle' is hidden in the letters of 'ballet'.
'alle' back-to-front is 'ELLA'.
(Other definitions for ella that I've seen before include "Diminutive of Eleanor" , ".... Fitzgerald, late great jazz diva" , "Ms Fitzgerald" , "Great late jazz singer . . . . Fitzgerald" , "Woman" .)