To sum up see no-one's left in ignorance (3,4)
I believe the answer is:
all told
'to sum' is the definition.
(I've seen this before)
'up see no-one's left in ignorance' is the wordplay.
I cannot quite see how this works, but
'no' could be 'o' (resembles 0 - 'no' can mean 'zero') and 'o' is present in the answer.
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for all told that I've seen before include "Taking everything into account" , "To summarise" , "Everyone informed" , "In summary" , "Nothing missed out" .)