The most common and most likely answer to this crossword clue is the 5 letter word TAXIS
I've seen this crossword clue in The Daily Telegraph
T
A
X
I
S
Buddy explains!

The straight part
We know that "transportation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case transportation = taxis.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.
What does TAXIS mean?
TAXIS noun- The directional movement of an organism in response to a stimulus.
- The manipulation of a body part into its normal position after injury.
- The arrangement of the parts of a topic.
- Arrangement or ordering generally, as in architecture or grammar
- A brigade in an Ancient Greek army.
- A vehicle that may be hired for single journeys by members of the public, driven by a taxi driver.
- A share taxi.
- To move an aircraft on the ground under its own power.
- To travel by taxicab.