Old journalist’s vitality, say, returning on vacation (5)
I believe the answer is:
pepys
'old journalist's' is the definition.
(diary-writer Samuel Pepys)
'vitality say returning on vacation' is the wordplay.
I cannot quite see how this works, but
'vitality' could be 'pep' and 'pep' is present in the answer.
This explanation may well be incorrect...
(Other definitions for pepys that I've seen before include "Famous English diarist" , "London diarist" , "Samuel, diarist, d. 1703" , "Samuel -, seventeenth century diarist" , "Samuel ---, C seventeen Admiralty secretary" .)