Journalist discovered it originally inside... (6)
I believe the answer is:
editor
'journalist' is the definition.
(senior journalist)
'discovered it originally inside' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'EDITOR' can be found hidden inside 'discovered it originally'.
(Other definitions for editor that I've seen before include "Director of 3 down" , "Chief journalist" , "Times boss maybe" , "Rag worker" , "Senior hack" .)