Mostly mock child showing a lack of sense (8)
I believe the answer is:
unreason
'mostly mock child' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'a lack of sense' is the wordplay.
'a lack of' becomes 'un' (I can't justify this - if you can you should give a lot more credence to this answer).
'sense' becomes 'reason' (I've seen this before).
'un'+'reason'='UNREASON'
'showing' acts as a link.
Can you help me to learn more?
(Other definitions for unreason that I've seen before include "nonsense" , "insanity" , "Irrationality" , "foolishness" , "The absence of rationality or logic" .)