The obscenely rich use it as a rule of thumb, perhaps (9)
I believe the answer is:
heuristic
'thumb perhaps' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'the obscenely rich use it as a rule' is the wordplay.
'rich use it' can be anagrammed to 'HEURISTIC'.
But, I'm unsure how this is indicated.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for heuristic that I've seen before include "Helping one to discover for oneself" , "classroom style?" .)