Mark's letter from Greece welcoming Turkish leader (6)
I believe the answer is:
stigma
'mark's' is the definition.
('stigma' can be a synonym of 'mark')
'letter from greece welcoming turkish leader' is the wordplay.
'letter from greece' becomes 'sigma' (letter from the Greek alphabet**).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'leader' says to take the initial letters.
The first letter of 'turkish' is 't'.
'sigma' enclosing 't' is 'STIGMA'.
(Other definitions for stigma that I've seen before include "Mark of shame or social disgrace" , "Bad mark" , "Symbol of disgrace or infamy" , "A stain as on character" , "Blot" .)