Levity? Forget it — this must be paid (4)
I believe the answer is:
levy
'this must be paid' is the definition.
(type of tax)
'levity? forget it' is the wordplay.
'forget' indicates named letters should be taken away.
'levity' with 'it' removed is 'LEVY'.
(Other definitions for levy that I've seen before include "Impose and collect (money)" , "Impost" , "Charge a fee or tax" , "Imposition of a tax" , "An imposed tax or duty" .)