Letters from Greece at river-mouths (6)
I believe the answer is:
deltas
This clue might be a double definition.
'letters' is the first definition.
(delta is a kind of letter)
'mouths' is the second definition.
(I have seen 'Wide mouths' mean 'deltas' so perhaps 'mouths' could also mean 'deltas')
'from greece at river' acts as a link.
This may not be right. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for deltas that I've seen before include "Triangular patches of land at mouths of rivers" , "Triangular areas where rivers divide before entering the sea" , "Lasted like triangular areas at river mouths" , "Triangular islands at mouths of rivers" , "Final sections of River" .)