Triangular Greek letters (6)
I believe the answer is:
deltas
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">triangular greek letters</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)