Thriller seen on Sunday is creating serious mental disorder (9)

I believe the answer is:

psychosis

I'm a little stuck... Click here to teach me more about this clue!

'mental disorder' is the definition.
(I've seen this before)

'thriller seen on sunday is creating' is the wordplay.
'thriller' becomes 'psycho' (I've seen this before).
'seen on' says to put letters next to each other.
'sunday' becomes 's'.
'creating' is an anagram indicator.
'is' anagrammed gives 'si'.
's' put after 'si' is 'sis'.
'psycho'+'sis'='PSYCHOSIS'

'serious' acts as a link.
This may not be correct. It may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for psychosis that I've seen before include "Severe mental disorder" , "madness" .)

I've seen this clue in the Sydney Morning Herald.

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