Discovered in voyage diary of old (4)
I believe the answer is:
aged
'old' is the definition.
(similar in meaning)
'discovered in voyage diary' is the wordplay.
'discovered' says the answer is hidden in the clue (remove the letters covering the answer in the clue).
'AGED' is hidden within 'in voyage diary'.
'of' is the link.
(Other definitions for aged that I've seen before include "Senior" , "Geriatric" , "Got older" , "Grew older" , "Senescent" .)