Show muscle in ruined castle (9)
I believe the answer is:
spectacle
'show' is the definition.
(thesaurus)
'muscle in ruined castle' is the wordplay.
'muscle' becomes 'pec' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'ruined' indicates anagramming the letters.
'castle' anagrammed gives 'stacle'.
'pec' put into 'stacle' is 'SPECTACLE'.
(Other definitions for spectacle that I've seen before include "Interesting phenomenon - show" , "Lavish entertainment" , "Entertaining sight, they help to see in the plural" , "public exhibition" , "Visually striking performance or display" .)