Fourth Greek letter (5)
I believe the answer is:
delta
'fourth greek letter' is the definition.
(I've seen this in another clue)
This is the entire 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" .)