Triangular Greek letters (6)
I believe the answer is:
deltas
'triangular greek letters' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for deltas that I've seen before include "Triangular land patches at river mouths" , "Lasted like triangular areas at river mouths" , "Wide mouths" , "Triangular patches of land at mouths of rivers" , "Triangular islands at mouths of rivers" .)