Fully avenged and that's flat! (4)
I believe the answer is:
even
This clue is a double definition.
'fully avenged' is the first definition.
(I've seen this in another clue)
'that's flat' is the second definition.
(level)
'and' acts as a link.
(Other definitions for even that I've seen before include "Not bumpy" , "happening to take time off?" , "Short joke" , "All square" , "Like 2, 4 or 6, say" .)