Flying ___ (geese formation) (3)
I believe the answer is:
vee
'flying geese formation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for vee that I've seen before include "Angled mark or notch" , "Letter; notch shape" , "22nd letter of the alphabet" , "Shape resembling the twenty-second letter" , "Character" .)