Said, by the way, the date had been altered (6)
I believe the answer is:
stated
'said' is the definition.
('state' can be a synonym of 'say')
'way the date had been altered' is the wordplay.
I cannot quite see how this works, but
'way' could be 'st' (short for street) and 'st' is found in the answer.
an anagram of 'date' is 'ated' which is present in the answer.
No letters remain.
This explanation may well be incorrect...
'by the' is the link.
Can you help me to learn more?
(Other definitions for stated that I've seen before include "Expressed in speech or writing" , "Expressed in written or spoken words" , "Declared or asserted" , "Said firmly, averred" , "Put in words, said" .)