No man is hiding in a castle! (7)
I believe the answer is:
mansion
'castle' is the definition.
(thesaurus)
'no man is hiding' is the wordplay.
'hiding' indicates anagramming the letters.
'no'+'man'+'is'='nomanis'
'nomanis' with letters rearranged gives 'MANSION'.
'in a' is the link.
(Other definitions for mansion that I've seen before include "Imposing house" , "Large dwelling" , "Huge imposing residence" , "Huge abode" , "large residence" .)