Bouncy castle collapses, trapping one (7)
I believe the answer is:
elastic
'bouncy' is the definition.
('elastic' can be similar in meaning to 'bouncy')
'castle collapses trapping one' is the wordplay.
'collapses' is an anagram indicator.
'trapping' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'castle' with letters rearranged gives 'elastc'.
'elastc' placed around 'i' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Stretcher" , "Castile (anag)" , "Able to give a bit" , "Giving" , "It's lace (anag.)" .)