To catcall twice is a mistake (6)
I believe the answer is:
booboo
'a mistake' is the definition.
(I've seen this before)
'to catcall twice' is the wordplay.
'to catcall' becomes 'boo' (I've seen this before).
'twice' shows that letters should be duplicated.
'boo' duplicated is 'BOOBOO'.
'is' acts as a link.
(Other definitions for booboo that I've seen before include "mistake" , "Having got animated, bear" , "Blunder" , "Error" .)