Pessimist needs reversal of gloomy outlook and organised rest (8)
I believe the answer is:
doomster
'pessimist needs reversal' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'gloomy outlook and organised rest' is the wordplay.
'gloomy outlook' becomes 'doom' (I can't justify this - if you can you should believe this answer much more).
'and' says to put letters next to each other.
'organised' indicates anagramming the letters.
'rest' anagrammed gives 'ster'.
'doom'+'ster'='DOOMSTER'
'of' is the link.
Can you help me to learn more?
(Other definitions for doomster that I've seen before include "In Ayr I used to say sentence" , "One predicting disaster" .)