She's up in class (3)
I believe the answer is:
sal
'she's' is the definition.
'up in class' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'in' indicates the answer is hidden within the clue.
'las' can be found hidden inside 'class'.
'las' back-to-front is 'SAL'.
(Other definitions for sal that I've seen before include "- - volatile" , "Sarah (dim.)" , "girl?" .)