Gamma follower (5)
I believe the answer is:
'gamma follower' 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 "Alluvial river mouth" , "Nile river mouth" , "Coastal feature" , "Roughly triangular area of alluvial deposits" , "Branching river mouth" .)