Bumped into one reversing the thing (4)
I believe the answer is:
item
'the thing' is the definition.
(thesaurus)
'bumped into one reversing' is the wordplay.
'bumped into' becomes 'met' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'reversing' says the letters should be written in reverse.
'met'+'i'='meti'
'meti' reversed gives 'ITEM'.
(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?" .)