Flexible individual initially visiting ruined castle (7)
I believe the answer is:
elastic
'flexible' is the definition.
(I know that flexible can be written as elastic)
'individual initially visiting ruined castle' is the wordplay.
'initially' indicates taking the first letters.
'visiting' is an insertion indicator.
'ruined' is an anagram indicator.
The initial letter of 'individual' is 'i'.
'castle' anagrammed gives 'elastc'.
'i' inserted within 'elastc' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Ductile" , "Flexible (material)" , "Flexible rubber strip" , "Resilient" , "capable of adapting to change?" .)