About a third of customers missed 7 21 (6)
I believe the answer is:
accuse
'third of customers missed 7 21' is the definition.
I can't tell whether this defines the answer.
'about a' is the wordplay.
I cannot really see how this works, but
'about' could be 'c' (abbreviation for 'circa') and 'c' is present in the answer.
'a' is within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for accuse that I've seen before include "Charge with wrong-doing" , "Impute fault to, indict" , "make charges" , "Put on trial" , "Apportion blame" .)