It's as much as you can do (6)
I believe the answer is:
utmost
'do' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'it's as much as you' is the wordplay.
I cannot really understand how this works, but
'you' could be 'u' (commonly used in online communication) and 'u' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'most' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'can' acts as a link.
Can you help me to learn more?
(Other definitions for utmost that I've seen before include "Level best" , "Maximum effort" , "'Most extreme, greatest (6)'" , "(To the) maximum extent" , "Farthest" .)