Bouncy castle melting around one (7)
I believe the answer is:
elastic
'bouncy' is the definition.
(similar in meaning)
'castle melting around one' is the wordplay.
'melting' is an anagram indicator.
'around' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'castle' with letters rearranged gives 'elastc'.
'elastc' going around 'i' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Fluid" , "Stretcher" , "Stretch material" , "Narrow strip of rubber" , "Kind of band" .)