What must be paid, lavish, expensive, vexes you at outset (4)
I believe the answer is:
levy
'what must be paid' is the definition.
(levy can mean a tax)
'lavish expensive vexes you at outset' is the wordplay.
'at outset' says to take the initial letters.
The first letters of 'lavish expensive vexes you' is 'LEVY'.
(Other definitions for levy that I've seen before include "Sum of money raised" , "tax money" , "Impose tax/fee/fine" , "Tax raised" , "draft" .)