The letter about me has since turned up (5)
I believe the answer is:
omega
'the letter' is the definition.
(omega is a kind of letter)
'about me has since turned up' is the wordplay.
'about' is an insertion indicator.
'me has' becomes 'em' (I can't explain this - if you can you should give a lot more credence to this answer).
'since' becomes 'ago' (I've seen this before).
'turned up' is a reversal indicator.
'em' inserted inside 'ago' is 'agemo'.
'agemo' written backwards gives 'OMEGA'.
Can you help me to learn more?
(Other definitions for omega that I've seen before include "Athenian character" , "Conclusion" , "24th Greek letter" , "long letter" , "Last thing" .)