Spoken, not written (4)
I believe the answer is:
oral
I believe this clue is a double definition.
'spoken' is the first definition.
(similar in meaning)
'not written' is the second definition.
(spoken rather than written)
(Other definitions for oral that I've seen before include "See 18" , "Spoken - examination" , "Relating to speech" , "By word of mouth (4)" , "Kind of exam" .)