Bouncy castle collapsed, engulfing one (7)
I believe the answer is:
elastic
'bouncy' is the definition.
(similar in meaning)
'castle collapsed engulfing one' is the wordplay.
'collapsed' indicates anagramming the letters.
'engulfing' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'castle' is an anagram of 'elastc'.
'elastc' enclosing 'i' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Ductile" , "Scale it (anag.)" , "Narrow band of rubber" , "Castile (anag)" , "Material for bands" .)