The things one’s bumped into backing in (5)
I believe the answer is:
items
'the things' is the definition.
(I've seen this before)
'one's bumped into backing in' is the wordplay.
'one's' becomes 'is' (roman numeral).
'bumped into' becomes 'met' (I've seen this before).
'backing' says the letters should be written in reverse.
'in' is an insertion indicator.
'met' written backwards gives 'tem'.
'is' going around 'tem' is 'ITEMS'.
(Other definitions for items that I've seen before include "Individual articles" , "Separate articles, as on list" , "Bits of news" , "separate parts of news bulletin" , "Couples" .)