Resilient one in ruined castle (7)
I believe the answer is:
elastic
'resilient' is the definition.
(similar in meaning)
'one in ruined castle' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'ruined' indicates an anagram.
'castle' is an anagram of 'elastc'.
'i' inserted inside 'elastc' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Something stretchy" , "Stretcher" , "Flexible, adaptable" , "Stretch material" , "Like one across" .)