Drink providing sustenance to Ulstermen, you might say? (9)
I believe the answer is:
orangeade
'drink' is the definition.
(I know that orangeade is a type of ade)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for orangeade that I've seen before include "Bubbly drink" , "Pop" , "Squash" , "Fruit drink" , "Soft citrus drink" .)