Take responsibility for a design blunder (3,4)
I believe the answer is:
own goal
'blunder' is the definition.
'take responsibility for a design' is the wordplay.
'take responsibility for' becomes 'own'.
'a design' becomes 'goal'.
'own'+'goal'='OWN GOAL'
(Other definitions for own goal that I've seen before include "Have a target" , "'Action that turns out bad for the person who took it (3,4)'" , "Self-damaging mistake" , "bad mistake" , "Action taken which results in a disadvantage to the taker" .)