The most common and most likely answer to this crossword clue is the 7 letter word BOLLARD
I've seen this crossword clue in The Daily Telegraph
B
O
L
L
A
R
D
Buddy explains!

The straight part
We know that "post" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case post = bollard.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 BOLLARD mean?
BOLLARD noun- A strong vertical post of timber or iron, fixed to the ground and/or on the deck of a ship, to which the ship's mooring lines etc are secured.
- A similar post preventing vehicle access to a pedestrian area, to delineate traffic lanes, or used for security purposes.