The most common and most likely answer to this crossword clue is the 4 plus 2 plus 3 plus 5 letter word FOAM AT THE MOUTH
F
O
A
M
A
T
T
H
E
M
O
U
T
H
More clues leading to the result FOAM AT THE MOUTH
Buddy explains!
The straight part
We know that "What a person having epileptic fit might do/become very angry" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what a person having epileptic fit might do/become very angry = foam at the mouth.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.