The most common and most likely answer to this crossword clue is the 16 letter word ROUND-SHOULDERED
I've seen this crossword clue in Financial Times
R
O
U
N
D
-
S
H
O
U
L
D
E
R
E
D
More clues leading to the result ROUND-SHOULDERED
Buddy explains!
The straight part
We know that "poor posture" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case poor posture = round-shouldered.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 ROUND-SHOULDERED mean?
ROUND-SHOULDERED adjective- Having a rounded upper back, and shoulders that bend forward.