Crossword Genius

Mostly mock child showing a lack of sense (8)

Ross

I believe the answer is:

unreason

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'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" .)

I've seen this clue in The Telegraph.
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