The most common and most likely answer to this crossword clue is the 4 plus 2 plus 3 plus 6 letter word SHIP OF THE DESERT
I've seen this crossword clue in The Guardian
S
H
I
P
O
F
T
H
E
D
E
S
E
R
T
More clues leading to the result SHIP OF THE DESERT
- Thief drops these off a dusty Land Cruiser
- 21 run off in pursuit of junk shop thief
- Transport speeds forth: he and I get bumped about, heading for terminus
- One with hump as these hopes drift sadly?
- Shopfitter heeds wild animal
- Fish, he protested, may evolve into an apparently marine creature
- Vital transport that took Lawrence aboard?
- This feted horse proverbially, in origin being designed — by a committee?
Buddy explains!
The straight part
We know that "21" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 21 = ship of the desert.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.