Bouncy castle collapsing round setter (7)
I believe the answer is:
elastic
'bouncy' is the definition.
('elastic' can be similar in meaning to 'bouncy')
'castle collapsing round setter' is the wordplay.
'collapsing' is an anagram indicator.
'round' indicates putting letters inside.
'setter' becomes 'i' (the person that set the crossword).
'castle' is an anagram of 'elastc'.
'elastc' enclosing 'i' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Stretchy knicker material" , "Laciest (anag.)" , "Narrow band of rubber" , "Able to give a bit" , "Stretch material" .)