Migratory formation (3)
I believe the answer is:
vee
'migratory 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 "22nd letter of the alphabet" , "Letter; notch shape" , "Shape resembling the twenty-second letter" , "Character" , "Angled mark or notch" .)