Fourth Greek letter (5)
I believe the answer is:
delta
'fourth greek letter' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for delta that I've seen before include "River mouth in the Greek alphabet" , "Branching river mouth" , "Area of distributaries at a river mouth" , "Comms code for 'd'" , "Triangular patch of land at mouth of river" .)