Bouncy castle collapsed around one (7)
I believe the answer is:
elastic
'bouncy' is the definition.
(similar in meaning)
'castle collapsed around one' is the wordplay.
'collapsed' indicates an anagram.
'around' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'castle' is an anagram of 'elastc'.
'elastc' going around 'i' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "capable of adapting to change?" , "Flexible rubber strip" , "Cats lie (anag.)" , "Stretching tape" , "Flexible rubbery strip" .)