Plenty must support vacant zombie fanatic (6)
I believe the answer is:
zealot
'fanatic' is the definition.
(I've seen this before)
'plenty must support vacant zombie' is the wordplay.
'plenty' becomes 'a lot' (a lot can mean plenty of something).
'must support' says to put letters next to each other (I've seen this in other clues).
'vacant' suggests removing the centre.
'zombie' with its middle taken out is 'ze'.
'alot' after 'ze' is 'ZEALOT'.
(Other definitions for zealot that I've seen before include "Fanatic or extreme enthusiast" , "Fervent advocate" , "Fervent, perhaps militant, proponent" , "Over-zealous person, fanatic" , "Fervent, even militant, proponent of something" .)