Horse fly seen around the central Nile (5)
I believe the answer is:
filly
'horse' is the definition.
(I have seen 'Female horse ' mean 'filly' so perhaps 'horse' could also mean 'filly')
'fly seen around the central nile' is the wordplay.
'seen around' indicates putting letters inside.
'the central' means to look at the middle letters.
The middle letters of 'nile' are 'il'.
'fly' placed around 'il' is 'FILLY'.
(Other definitions for filly that I've seen before include "Young mare" , "Female horse under four years old" , "Young female horse" .)