Saw Twitter introducing a letter signifying unknown (6)
I believe the answer is:
saying
'saw twitter' is the definition.
'saying' can be an answer for 'saw' (a saw can mean a proverb or maxim). I'm not sure about the 'twitter' bit.
'introducing a letter signifying unknown' is the wordplay.
'introducing' indicates putting letters inside (some letters introduced or inserted into others).
'letter' becomes 'y' (Y is a letter of the alphabet).
'signifying unknown' becomes 'sing' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'y'='ay'
'ay' placed into 'sing' is 'SAYING'.
Can you help me to learn more?
(Other definitions for saying that I've seen before include "Speaking; adage" , "Apophthegm" , "Motto" , "Epigram" , "Adage, maxim" .)