Getting this upset may cause bother
I believe the answer is:
applecart
This might be a double definition.
'getting this upset' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'bother' is the second definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'may cause' acts as a link.
Can you help me to learn more?
(Other definitions for applecart that I've seen before include "one that carries fruit" , "Spoil carefully laid plans - upset the . . . . . . . . ." , "Upset this fruit conveyance to spoil plans" , "Upset this fruit carrier to spoil plans" , "transport for cookers, etc" .)