They fly east-southeast at some stage (5)
I believe the answer is:
geese
'they fly' is the definition.
Although both the answer and 'fly' are nouns, they are not in the same form of the noun.
'east-southeast at some stage' is the wordplay.
'east southeast' becomes 'ESE' (synonyms).
'at' says to put letters next to each other (I've seen this in other clues).
'some stage' becomes 'ge' (I can't explain this - if you can you should believe this answer much more).
'ese' put after 'ge' is 'GEESE'.
Can you help me to learn more?
(Other definitions for geese that I've seen before include "Ganders" , "Simpletons" , "Long-necked, migratory aquatic birds" , "Edible water birds" , "Birds that fly in a skein" .)