Received article thanks to a letter from Greece (5)
I believe the answer is:
theta
'a letter from greece' is the definition.
(letter from the Greek alphabet)
'received article thanks' is the wordplay.
'received article' becomes 'the' (the definite article. I am not sure about the 'received' bit.).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'the'+'ta'='THETA'
'to' is the link.
Can you help me to learn more?
(Other definitions for theta that I've seen before include "character in Marathon" , "Foreign letter" , "classic character?" , "Foreign character" , "Eighth letter of Greek alphabet" .)