To do up a tidy bundle is something abnormal (6)
I believe the answer is:
oddity
'something abnormal' is the definition.
I can't tell whether this defines the answer.
'to do up a tidy bundle' is the wordplay.
'to do up' becomes 'od' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bundle' indicates anagramming the letters.
'tidy' anagrammed gives 'dity'.
'od'+'dity'='ODDITY'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for oddity that I've seen before include "Strange person/thing" , "Twelve months" , "Quirk" , "Quality of strangeness" , "Curious item" .)