He's apt to be first in alphabetical order (5)
I believe the answer is:
aaron
'order' is the definition.
(I know that Aaron is a Priest)
'he's apt to be first in alphabetical' is the wordplay.
'he's apt' becomes 'aron' (I can't explain this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'be first in alphabetical' becomes 'a' (ast letter of 'alphabetical').
'aron' after 'a' is 'AARON'.
Can you help me to learn more?
(Other definitions for aaron that I've seen before include "Brother of Moses and traditional founder of Jewish priesthood" , "- - Copland (mus.)" , "OT character" , "Elder brother of Moses" , "leader's brother" .)