Crossword Genius

The obscenely rich use it as a rule of thumb, perhaps (9)

Ross

I believe the answer is:

heuristic

I'm a little stuck... Click here to teach me more about this clue!

'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?" .)

I've seen this clue in The Guardian.
Want a hint initially instead of a full solution? Install my app