One having no illusions about most celebrated group of individuals (7)
I believe the answer is:
realist
'one having no illusions' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'about most celebrated group of individuals' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'most celebrated group of individuals' becomes 'list' (I can't justify this - if you can you should give a lot more credence to this answer).
'rea'+'list'='REALIST'
Can you help me to learn more?
(Other definitions for realist that I've seen before include "who may look for practical solution" , "Practical sort" , "Pragmatist" , "Pragmatic person" , "Person who sees things without illusions" .)