Letter from Greece about introduction of this brand (6)
I believe the answer is:
stigma
'brand' is the definition.
('stigma' can be a synonym of 'brand')
'letter from greece about introduction of this' is the wordplay.
'letter from greece' becomes 'sigma' (letter from the Greek alphabet).
'about' means one lot of letters goes inside another.
'introduction of' suggests taking the first letters.
The first letter of 'this' is 't'.
'sigma' going around 't' is 'STIGMA'.
(Other definitions for stigma that I've seen before include "Mark of shame or social disgrace" , "Disgraceful mark" , "part of plant" , "Mark of dishonour" , "Symbol of disgrace or infamy" .)