Fantastic castle turned up: item at auction (7)
I believe the answer is:
camelot
'fantastic castle' is the definition.
The definition suggests a singular noun which matches the answer.
'turned up item at auction' is the wordplay.
'turned up' becomes 'came' (I've seen this in another clue).
'item at auction' becomes 'lot' (lot can mean an item up for sale at an auction).
'came'+'lot'='CAMELOT'
Can you help me to learn more?
(Other definitions for camelot that I've seen before include "Where truth, goodness and beauty reigned, according to Arthurian legend" , "Mythical British kingdom" , "King Arthur's court - or JFK's" , "Legendary court" , "King Arthur's town" .)