Thing, that, reversing, ran into one (4)
I believe the answer is:
item
'thing' is the definition.
(thesaurus)
'reversing ran into one' is the wordplay.
'reversing' says the letters should be written in reverse.
'ran into' becomes 'met'.
'one' becomes 'i' (Roman numeral).
'met'+'i'='meti'
'meti' back-to-front is 'ITEM'.
'that' is the link.
(Other definitions for item that I've seen before include "Particular" , "Detail - entry" , "Feature" , "story" , "A single piece of information" .)