Not fantastic entertaining Duke's circle at first - horrible experience! (6)
I believe the answer is:
ordeal
'horrible experience' is the definition.
(I've seen this before)
'not fantastic entertaining duke's circle at first' is the wordplay.
'not fantastic' becomes 'real' (I can't explain this - if you can you should give a lot more credence to this answer).
'entertaining' means one lot of letters goes inside another.
'duke' becomes 'd'.
'circle' becomes 'o'.
'at first' says to put letters next to each other.
'real' going around 'd' is 'rdeal'.
'rdeal' after 'o' is 'ORDEAL'.
Can you help me to learn more?
(Other definitions for ordeal that I've seen before include "Torment" , "Severe or trying experience" , "Prolonged distressing experience" , "Difficult and testing experience" , "Trial; nightmare" .)