Pepys, for one, is said to be the worst (6)
I believe the answer is:
direst
'worst' is the definition.
(I've seen this in another clue)
'pepys for one is said to be' is the wordplay.
'pepys for one' becomes 'diarist' (Samuel Pepys was a famous diarist).
'is said to be' shows a homophone (sound like).
'diarist' sounds like 'DIREST'.
'the' is the link.
(Other definitions for direst that I've seen before include "Most horrific" , "far from ideal" .)