She upsets us at closing time (3)
I believe the answer is:
sue
'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'.
'su'+'e'='SUE'
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" .)