Diarist has a furtive look, reportedly (5)
I believe the answer is:
'diarist' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for pepys that I've seen before include "Renowned diarist", "Samuel -, English diarist", "English diarist and naval administrator, d. 1703", "Famous English diarist", "Diary-writer".)