"Keep Climbing" airline (5)
I believe the answer is:
delta
'keep climbing airline' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for delta that I've seen before include "Fourth letter of the ancient Greek alphabet: river feature" , "Area of distributaries at a river mouth" , "Multiple estuary" , "Wide silted river mouth" , "The Greek D (5)" .)