The most common and most likely answer to this crossword clue is the 17 letter word MANIC-DEPRESSIVES
I've seen this crossword clue in The Guardian
M
A
N
I
C
-
D
E
P
R
E
S
S
I
V
E
S
Buddy explains!
The straight part
We know that "people with severe mood swings" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case people with severe mood swings = manic-depressives.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.