Fanatic has seconds of quality treat (5)
I believe the answer is:
ultra
'fanatic' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ultra that I've seen before include "Over the top" , "Fanatic" , "Violet's predecessor" , "such sound treatment?" , "Secret intelligence" .)