Document I don't know, left at sea (8)
I believe the answer is:
passport
'document' is the definition.
(passport is a kind of document)
'i don't know left at sea' is the wordplay.
'i don't know' becomes 'pass' (e.g. response in a quiz).
'left at sea' becomes 'port' (I've seen this in another clue).
'pass'+'port'='PASSPORT'
(Other definitions for passport that I've seen before include "Carte blanche" , "Traveller's aid" , "Travel doc" , "the key to success?" , "Authorization to go abroad and return" .)