Old rich Tories drunk on power (11)
I believe the answer is:
prehistoric
'old' is the definition.
(thesaurus)
'rich tories drunk on power' is the wordplay.
'drunk' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'power' becomes 'p' (symbol in physics).
'rich'+'tories'='richtories'
'richtories' anagrammed gives 'rehistoric'.
'rehistoric' put after 'p' is 'PREHISTORIC'.
(Other definitions for prehistoric that I've seen before include "Before written records" , "Extremely old" , "antiquated" , "Very old or out of date" , "Ancient" .)