Go some days as a diarist (5)
I believe the answer is:
pepys
'a diarist' is the definition.
(I've seen this before)
'go some days' is the wordplay.
'go' becomes 'pep' (both can mean energy or vitality).
'some days' becomes 'ys' (I can't explain this - if you can you should give a lot more credence to this answer).
'pep'+'ys'='PEPYS'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for pepys that I've seen before include "London diarist" , "Diary-writer" , "Famous diary keeper" , "Samuel, diarist, d. 1703" , "English diarist and naval administrator, d. 1703" .)