Letter from Greece about Turkish leader's disgrace (6)
I believe the answer is:
stigma
'disgrace' is the definition.
(I've seen this in another clue)
'letter from greece about turkish leader's' is the wordplay.
'letter from greece' becomes 'sigma' (letter from the Greek alphabet).
'about' means one lot of letters goes inside another.
'leader's' indicates taking the first letters (I've seen 'leaders' mean this).
The first letter of 'turkish' is 't'.
'sigma' enclosing 't' is 'STIGMA'.
(Other definitions for stigma that I've seen before include "Brand or mark of infamy" , "Blot" , "Mark of dishonour" , "Sign of disgrace" , "Bad mark" .)