Ultimately trust men involved in robbery, I suppose (8)
I believe the answer is:
theorist
'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" .)