The most common and most likely answer to this crossword clue is the 10 letter word NIGHTSHIRT
I've seen this crossword clue in The Daily Telegraph
N
I
G
H
T
S
H
I
R
T
More clues leading to the result NIGHTSHIRT
Buddy explains!
The straight part
We know that "Mister finally wears this thing possibly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mister finally wears this thing possibly = nightshirt.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 NIGHTSHIRT mean?
NIGHTSHIRT noun- A shirt-like garment (often oversized) that is worn to bed.