She upsets us at closing time (3)
I believe the answer is:
'she' is the definition.
'upsets us at closing time' is the wordplay.
'upsets' is a reversal indicator.
'at' says to put letters next to each other.
'closing time' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'us' in reverse letter order is 'su'.
Can you help me to learn more?
(Other definitions for sue that I've seen before include "The girl to take to court" , "Take proceedings against (a person)" , "Charge" , "Make a legal claim" , "Bring charges against - girl" .)