A Parisian's word upsetting to Monsieur (3)
I believe the answer is:
mot
'a parisian's word' is the definition.
('word' in French)
'upsetting to monsieur' is the wordplay.
'upsetting' is a reversal indicator.
'monsieur' becomes 'm'.
'to'+'m'='tom'
'tom' back-to-front is 'MOT'.
(Other definitions for mot that I've seen before include "witty remark" , "French word for Tom" , "type of test" , "Word in France, bon perhaps" , "Witty saying" .)