Logic that ignores an upset (9)
I believe the answer is:
reasoning
'logic' is the definition.
(thesaurus)
'ignores an upset' is the wordplay.
'upset' indicates anagramming the letters.
'ignores'+'an'='ignoresan'
'ignoresan' anagrammed gives 'REASONING'.
'that' is the link.
(Other definitions for reasoning that I've seen before include "Thinking that is coherent and logical" , "Logic" , "Train of thought" , "Logical thought process" , "Debating" .)