A castle is a chess piece; denial is preposterous (7)
I believe the answer is:
mansion
'a castle' is the definition.
(thesaurus)
'chess piece denial is preposterous' is the wordplay.
I cannot quite understand how this works, but
'piece' could be 'ans' (ANS is an example) and 'ans' is found within the answer.
This explanation may well be incorrect...
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for mansion that I've seen before include "Big house" , "Imposing house" , "Manor house" , "Huge abode" , "Huge imposing residence" .)