Supporter of dumb rationales (3)
I believe the answer is:
bra
'supporter' is the definition.
(thesaurus)
'of dumb rationales' is the wordplay.
'of' indicates a hidden word.
'BRA' can be found hidden inside 'dumb rationales'.
(Other definitions for bra that I've seen before include "Chest supporter" , "Female undergarment" , "Underwear item" , "support" , "I give a couple a lift" .)