Mistake to express disapproval twice (6)
I believe the answer is:
booboo
'mistake' is the definition.
(informal term for a mistake)
'disapproval twice' is the wordplay.
'disapproval' becomes 'boo' (as in a crowd booing).
'twice' says to double the letters.
'boo' doubled is 'BOOBOO'.
'to express' is the link.
(Other definitions for booboo that I've seen before include "mistake" , "Blunder" , "Error" , "Having got animated, bear" .)