Fourth letter of the Greek alphabet (5)
I believe the answer is:
delta
'fourth letter of the greek alphabet' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for delta that I've seen before include "Fourth Greek letter? - area at river mouth" , "Triangular patch of land at mouth of river" , "Daedalus started with this" , "The fourth Greek letter found at river's mouth" , "Foreign character" .)