Crossword Genius

Guideline set by Tom when in charge? (4,2,5)


I believe the answer is:

rule of thumb

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

'guideline' is the definition.

'set by tom when in charge?' is the wordplay.
I cannot quite see how this works, but
'by' could be 'of' and 'of' is present in the answer.
'tom' could be 'thumb' (folklore character Tom Thumb) and 'thumb' is found within the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for rule of thumb that I've seen before include "Practical guidance" , "Ad hoc form of gauging something" , "digital governance?" , "Practical guide" , "apparent when Tom was in charge" .)

I've seen this clue in The Telegraph.
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