Resilient one in crumbling castle (7)
I believe the answer is:
elastic
'resilient' is the definition.
(similar in meaning)
'one in crumbling castle' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'crumbling' indicates an anagram.
'castle' is an anagram of 'elastc'.
'i' going inside 'elastc' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Something stretchy" , "Variable" , "'Flexible, springy (7)'" , "Cats lie (anag.)" , "Able to give a bit" .)