Woman in Madrid’s preposterous reason (6)
I believe the answer is:
senora
'woman in madrid's' is the definition.
(term of address for a woman in Spanish)
'preposterous reason' is the wordplay.
'preposterous' indicates anagramming the letters.
'reason' with letters rearranged gives 'SENORA'.
(Other definitions for senora that I've seen before include "Spanish title" , "Lady of Spain" , "Spanish lady within reason" , "Reason with a Spanish woman, probably married" , "Reason with Spanish Mrs" .)