Impossible to find who’s originally featured in drama once before (7)
I believe the answer is:
nowhere
'impossible to find' is the definition.
(as in 'nowhere to be found')
'who's originally featured in drama once before' is the wordplay.
'originally' says to take the initial letters.
'featured in' indicates putting letters inside.
'drama' becomes 'noh' (Noh or No is a Japanese masked drama).
'once before' becomes 'ere' (archaic term for 'before').
The first letter of 'who' is 'w'.
'w' inserted inside 'noh' is 'nowh'.
'nowh'+'ere'='NOWHERE'
(Other definitions for nowhere that I've seen before include "insignificant place?" , "in limbo" , "Nonexistent place" , "miles down the field" , "News from - (William Morris)" .)