Spot when king leaves castle -- fantastic things worth looking at (10)
I believe the answer is:
spectacles
'things worth looking at' is the definition.
'spot when king leaves castle fantastic' is the wordplay.
I cannot quite see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
an anagram of 'castle' is 'tacles' which is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for spectacles that I've seen before include "Aid to vision" , "splendid entertainments?" , "With which to view" , "(Pair of) glasses" , "See 6" .)