The most common and most likely answer to this crossword clue is the 2 plus 3 plus 4 letter word DO NOT BEND
I've seen this crossword clue in The Times
D
O
N
O
T
B
E
N
D
Buddy explains!
The straight part
We know that "Warning to post-holder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case warning to post-holder = do not bend.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.