If you've done this, there's something you just won't get (3,2)

'you just won't get' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'if you've done this there's something' is the wordplay.
I cannot really see how this works, but
'done' could be 'had' ('do' and 'have' can both mean to defraud) and 'had' is present in the answer.
'something' could be 'it' (both can refer to an object) and 'it' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?