Crossword Genius

Ultimately trust men involved in robbery, I suppose (8)

Ross

I believe the answer is:

theorist

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

'ultimately trust' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'men involved in robbery i suppose' is the wordplay.
I cannot quite understand how this works, but
'men' could be 'or' (military abbreviation for Other Ranks) and 'or' is found in the answer.
'robbery' could be 'heist' (heist is a kind of robbery) and 'heist' is located in the remaining letters.
A single letter 't' remains which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for theorist that I've seen before include "Abstract reasoner" , "An expert in abstract principles" , "Expert in established hypotheses" , "Speculator" , "Thinker" .)

I've seen this clue in The Times.
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