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')
'the way the date had been altered' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'way' could be 's' (Compass direction - South) and 's' is found within the answer.
an anagram of 'date' is 'ated' which is present in the answer.
No letters remain.
This explanation may well be incorrect...
'by' 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" .)