This uses a charade technique - combining parts from different words in the clue to build the answer.
Look at 'Offer and appraisal both' - you need to extract specific letters from each of these words. 'Turned out to be' indicates how these letters combine to form the answer.
Answer: ORAL | Take the first two letters from 'Offer' (OR) and the first two letters from 'appraisal' (AL) to get ORAL, which means unwritten (spoken rather than written).