They believe nothing in this is real ultimately, somehow (9)
I believe the answer is:
nihilists
'they believe nothing' is the definition.
(nihilists believe everything is meaningless)
'in this is real ultimately somehow' is the wordplay.
'ultimately' indicates one should take the final letters.
'somehow' indicates anagramming the letters.
The final letter of 'real' is 'l'.
'in'+'this'+'is'+'l'='inthisisl'
'inthisisl' is an anagram of 'NIHILISTS'.
(Other definitions for nihilists that I've seen before include "Old revolutionaries" , "unbelievers" , "They reject all theories of morality or religious belief" .)