Letter to Greek duke terribly late (5)
I believe the answer is:
delta
'letter' is the definition.
(Greek letter)
'greek duke terribly late' is the wordplay.
'greek duke' becomes 'd' (abbreviation. I am not sure about the 'greek' bit.).
'terribly' is an anagram indicator.
'late' with letters rearranged gives '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 "Coastal feature" , "Triangular patch of land at mouth of river" , "sort of wing" , "Multiple estuary" , "Estuary area where river has split" .)