The most common and most likely answer to this crossword clue is the 8 plus 7 letter word DRESSING STATION
I've seen this crossword clue in The Times
D
R
E
S
S
I
N
G
S
T
A
T
I
O
N
Buddy explains!
The straight part
We know that "First aid post" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case first aid post = dressing station.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.