Bumped into one reversing the thing (4)
I believe the answer is:
item
'thing' is the definition.
(an item is a thing)
'bumped into one reversing' is the wordplay.
'bumped into' becomes 'met' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'reversing' is a reversal indicator.
'met'+'i'='meti'
'meti' back-to-front is 'ITEM'.
'the' is the link.
(Other definitions for item that I've seen before include "Seperate article in an enumeration" , "Two lovers as one" , "'One thing, poor mite (4)'" , "One thing - two people, perhaps" , "Two people widely seen as one?" .)