Area around the mouth (5)
I believe the answer is:
delta
'area around the mouth' 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 "Code word for D" , "Area of alluvial deposit" , "Fourth Greek letter? - area at river mouth" , "The Greek D (5)" , "first used by Daedalus" .)