Depth unknown, doctor found them a problem (10)

I believe the answer is:

unfathomed

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

'depth unknown doctor' is the definition.
I can't judge whether this defines the answer.

'found them a problem' is the wordplay.
'problem' is an anagram indicator.
'found'+'them'+'a'='foundthema'
'foundthema' anagrammed gives 'UNFATHOMED'.

Can you help me to learn more?

(Other definitions for unfathomed that I've seen before include "Never sounded" , "Bottomless (as far as we know)" .)

I've seen this clue in the Sydney Morning Herald.

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