The thing I bumped into, turning (4)
I believe the answer is:
item
'the thing' is the definition.
(thesaurus)
'i bumped into turning' is the wordplay.
'bumped into' becomes 'met' (I've seen this before).
'turning' says the letters should be written in reverse.
'met' back-to-front is 'tem'.
'i'+'tem'='ITEM'
(Other definitions for item that I've seen before include "'One thing, poor mite (4)'" , "Two lovers as one" , "pair?" , "Romantic couple" , "Detail - entry" .)