Not even mentally unstable (10)

I believe the answer is:

unbalanced

I'm a little stuck... Click here to teach me more about this clue!

'mentally unstable' is the definition.
'unbalanced' can be an answer for 'unstable' (thesaurus). I'm not certain of the 'mentally' bit.

'not even' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'even' becomes 'balanced' (I've seen this before).
'un'+'balanced'='UNBALANCED'

Can you help me to learn more?

(Other definitions for unbalanced that I've seen before include "Wanting fairness" , "not put on scales?" , "Nuts" .)

I've seen this clue in The Telegraph.

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