De Quincey, say, by his own admission, concocted mere Utopia (5-5)
I believe the answer is:
opium-eater
'de quincey say by his own admission' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'concocted mere utopia' is the wordplay.
'concocted' indicates anagramming the letters.
'mere'+'utopia'='mereutopia'
'mereutopia' with letters rearranged gives 'OPIUM-EATER'.
Can you help me to learn more?
(Another definition for opium-eater that I've seen is " Confessions of an English -- (de Quincey)".)