In general it's marvellous they won't accept sum for redesign (7)
I believe the answer is:
overall
'in general' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for overall that I've seen before include "Boilersuit" , "generally speaking" , "on average" , "Protective coat painters put on" , "Protective garment worn over ordinary clothes" .)