Drunk let aboard car? It's a problem for traveller (3,3)
I believe the answer is:
jet lag
'a problem for traveller' is the definition.
(problem when moving time zones)
'drunk let aboard car?' is the wordplay.
'drunk' is an anagram indicator.
'aboard' indicates putting letters inside.
'car?' becomes 'Jag' (short for Jaguar).
'let' is an anagram of 'etl'.
'etl' put within 'jag' is 'JET-LAG'.
'it's' is the link.
(Other definitions for jet lag that I've seen before include "Discomfort due to changing time zones when flying" , "Body's disruption after a long flight" , "Effect of long distance journey" , "Fatigue after a long-haul flight" , "Tiredness felt after long plane journey" .)