Permit to go by on the left (8)
I believe the answer is:
passport
'permit' is the definition.
(passport is a type of permit)
'go by on the left' is the wordplay.
'go by' becomes 'pass' (synonyms).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the left' becomes 'port' (left side of a ship - opposite of starboard).
'pass'+'port'='PASSPORT'
'to' acts as a link.
(Other definitions for passport that I've seen before include "papers" , "you might need Visa with this" , "entry permit?" , "one admits" , "Official document required for international travel" .)