This admission to castle is a let-down (10)
I believe the answer is:
drawbridge
'this admission' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'castle is a let-down' is the wordplay.
I cannot quite see how this works, but
'castle' could be 'r' (abbreviation for rook/castle in chess) and 'r' is located in the answer.
'a' is present in the answer.
'down' could be 'd' and 'd' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for drawbridge that I've seen before include "Keep attachment" , "Castle defence" , "One may protect castle" , "that's raised by defenders" , "Barbican may defend this" .)