The most common and most likely answer to this crossword clue is the 4 plus 4 letter word ROOF RACK
I've seen this crossword clue in The Guardian
R
O
O
F
R
A
C
K
More clues leading to the result ROOF RACK
- Attachment OK for car when moving
- One assisting in transporting stone boxes from Argentina
- OK for car to be adapted — it’s for carrying more luggage
- Evidence trail leads off to where traveller’s cases are held
- More space for luggage found by fellow reversing vehicle into castle
- Frame OK for car when travelling
- Vehicle accessory surprisingly OK for car
- Vehicle attachment, sort of bar to extract oil?
- Luggage carrier OK for car if modified
- Carrier loosely OK for car
Buddy explains!
The straight part
We know that "where traveller’s cases are held" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where traveller’s cases are held = roof rack.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.