Letter to Greek duke rewritten late (5)
I believe the answer is:
delta
'letter' is the definition.
(Greek letter)
'greek duke rewritten late' is the wordplay.
'greek duke' becomes 'd' (abbreviation. I am not sure about the 'greek' bit.).
'rewritten' indicates anagramming the letters.
'late' is an anagram of 'elta'.
'd'+'elta'='DELTA'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for delta that I've seen before include "See Greek" , "Area of alluvial deposit" , "Alluvial river mouth" , "The Greek D (5)" , "Dealt (anag.)" .)