Immediately after that, he threw it out (9)
I believe the answer is:
therewith
'immediately after that' is the definition.
(I've seen this before)
'he threw it out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'he'+'threw'+'it'='hethrewit'
'hethrewit' anagrammed gives 'THEREWITH'.
(Other definitions for therewith that I've seen before include "Using that" , "accompanying it" , "Immediately after that" , "Soon after that" .)