The most common and most likely answer to this crossword clue is the 7 plus 11 plus 9 letter word LINCOLN CONTINENTAL LIMOUSINE
I've seen this crossword clue in The Guardian
L
I
N
C
O
L
N
C
O
N
T
I
N
E
N
T
A
L
L
I
M
O
U
S
I
N
E
Buddy explains!
The straight part
We know that "I’m stretched out, tired and exhausted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i’m stretched out, tired and exhausted = lincoln continental limousine.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.