Atlanta-based airline (5)
I believe the answer is:
delta
'atlanta-based airline' 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" , "Eg, outlet of the Nile" , "Multiple estuary" , "Letter - where river divides" , "Branching river mouth" .)