There's trouble when this is left on travel (8)
I believe the answer is:
passport
'there's trouble when this' is the definition.
The definition suggests a singular noun which matches the answer.
'left on travel' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'on' is a charade indicator (letters next to each other).
'travel' becomes 'pass' (passing is a kind of travelling**).
'port' after 'pass' is 'PASSPORT'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for passport that I've seen before include "Traveller's ID" , "Official document required for international travel" , "Travel identification" , "Travel ID" , "you might need Visa with this" .)