One can't do more than this (6)
I believe the answer is:
utmost
'one' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?
'can't do more than this' is the wordplay.
I cannot really see how this works, but
'do' could be 'ut' (synonyms) and 'ut' is found in the answer.
'more' could be 'mo' (I've seen this in other clues) and 'mo' is found within the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for utmost that I've seen before include "Extremest" , "Finest" , "The greatest possible degree" , "(To the) greatest possible extent" , "Level best" .)