Tax frivolity, dismissing it (4)
I believe the answer is:
levy
'tax' is the definition.
(thesaurus)
'frivolity dismissing it' is the wordplay.
'frivolity' becomes 'levity' (levity is a kind of frivolity).
'dismissing' indicates named letters should be taken away.
'levity' with 'it' removed is 'LEVY'.
(Other definitions for levy that I've seen before include "tax money" , "Impose and collect (money)" , "Imposition of a tax" , "Impose a tax, fine, etc" , "Money raised from taxes, etc" .)