Ruined castle collapsed trapping one (7)
I believe the answer is:
elastic
'ruined' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'castle collapsed trapping one' is the wordplay.
'collapsed' indicates an anagram.
'trapping' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'castle' with letters rearranged gives 'elastc'.
'elastc' placed around 'i' is 'ELASTIC'.
Can you help me to learn more?
(Other definitions for elastic that I've seen before include "Kind of band" , "Resilient" , "Tape containing flexible rubber" , "Narrow strip of rubber" , "Pliable" .)