Try one's level best to swindle as many as possible (2,3,3,3)
I believe the answer is:
do all one can
'try' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'one's level best to swindle as many as possible' is the wordplay.
I cannot quite understand how this works, but
'level' could be 'o' (O level is an example) and 'o' is present in the answer.
'best' could be 'a' and 'a' is present in the answer.
'many' could be 'l' and 'l' is located in the answer.
'possible' could be 'on' ('not on' can mean unacceptable or not possible) and 'on' is located in the answer.
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for do all one can that I've seen is " Try one's hardest".)