Leave out VAT? Ace! (6)
I believe the answer is:
vacate
'leave' is the definition.
(vacating is a kind of leaving)
'out vat? ace' is the wordplay.
'out' indicates an anagram.
'vat'+'ace'='vatace'
'vatace' is an anagram of 'VACATE'.
(Other definitions for vacate that I've seen before include "Clear out and leave empty" , "Withdraw as from premises" , "Leave (a position)" , "Make empty" , "Quit" .)