A number are surrounded by darting horse flies (6)
I believe the answer is:
hovers
'flies' is the definition.
(hovering is a kind of flying)
'a number are surrounded by darting horse' is the wordplay.
'a number' becomes 'v' (V is an example).
'are surrounded by' means one lot of letters goes inside another.
'darting horse' becomes 'hoers' (I can't justify this - if you can you should believe this answer much more).
'v' placed inside 'hoers' is 'HOVERS'.
Can you help me to learn more?
(Other definitions for hovers that I've seen before include "Floats above" , "Hangs in the air without moving" , "Hangs in the air (unable to decide?)" .)