R&A plot to overturn ban (5)
I believe the answer is:
debar
'ban' is the definition.
(thesaurus)
'ra plot to overturn' is the wordplay.
'plot' becomes 'bed' (bed is a kind of plot).
'to overturn' says the letters should be written in reverse.
'ra'+'bed'='rabed'
'rabed' in reverse letter order is 'DEBAR'.
(Other definitions for debar that I've seen before include "Exclude from a place" , "Large advert" , "Shut out, preclude" , "Keep out" , "Refuse to admit" .)