Not a mistake made though being dim? (7,5)
I believe the answer is:
glaring error
'though being dim?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'not a mistake' is the wordplay.
'not' becomes 'glaring' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a mistake' becomes 'error' ('error' can be a synonym of 'mistake').
'glaring'+'error'='GLARING ERROR'
'made' acts as a link.
Can you help me to learn more?
(Another definition for glaring error that I've seen is " Conspicuous mistake".)