Expressions of disapproval must be a mistake (6)
I believe the answer is:
booboo
'a mistake' is the definition.
(I've seen this before)
'expressions of disapproval' is the wordplay.
'expressions of' becomes 'boo' (I can't explain this - if you can you should give a lot more credence to this answer).
'disapproval' becomes 'boo' (I have seen 'Express disapproval ' mean 'boo' so perhaps 'disapproval' could also mean 'boo').
'boo'+'boo'='BOOBOO'
'must be' is the link.
Can you help me to learn more?
(Other definitions for booboo that I've seen before include "Error" , "Blunder" , "Having got animated, bear" , "mistake" .)