Document I don't know on drink (8)
I believe the answer is:
passport
'document' is the definition.
(passport is a kind of document)
'i don't know on drink' is the wordplay.
'i don't know' becomes 'pass' (I've seen this before).
'on' says to put letters next to each other.
'drink' becomes 'port' (port is a kind of drink).
'pass'+'port'='PASSPORT'
(Other definitions for passport that I've seen before include "Travel authorisation" , "entry permit?" , "Document facilitating travel" , "Authorization to go abroad and return" , "papers" .)