A vote against Brexit had to rule out every border disorder (8)
I believe the answer is:
anorexia
'disorder' is the definition.
(anorexia is a kind of disorder)
'a vote against brexit had to rule out every border' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
'vote' could be 'x' (how votes are marked) and 'x' is located in the answer.
'rule' could be 'r' (legal abbreviation) and 'r' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for anorexia that I've seen before include "Obsessive desire to lose weight with refusal to eat normally" , "Clinical lack of appetite, wasting ailment" , "An abnormal condition that affects and inhibits eating" , "Food-refusing disorder" , "Psychological condition causing reluctance to eat" .)