Not mentioned (being unspeakable?) (6)
I believe the answer is:
unsaid
'unspeakable?' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'not mentioned' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'mentioned' becomes 'said' (I've seen this in another clue).
'un'+'said'='UNSAID'
'being' acts as a link.
Can you help me to learn more?
(Other definitions for unsaid that I've seen before include "Not uttered" , "not delivered" , "with lack of expression" , "Not made explicit" , "omitted from speech" .)