Elizabeth's dad and mother left in convulsive form of epilepsy (5,3)

'epilepsy' is the definition.
(I know that grand mal is a type of epilepsy)

'elizabeth's dad and mother left' is the wordplay.
'elizabeth's dad' becomes 'grand' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'left' becomes 'L' (common abbreviation).
'grand'+'ma'+'l'='GRAND MAL'

'in convulsive form of' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?