Express disapproval of bishop? That would be a mistake (4)
I believe the answer is:
boob
'a mistake' is the definition.
(I've seen this before)
'express disapproval of bishop?' is the wordplay.
'express disapproval of' becomes 'boo' (I've seen this before).
'bishop?' becomes 'B' (chess abbreviation).
'boo'+'b'='BOOB'
'that would be' acts as a link.
(Other definitions for boob that I've seen before include "Error" , "Embarrassing mistake" , "Slip" , "Blunder (colloq.)" , "Mistake; breast (sl.)" .)