The most common and most likely answer to this crossword clue is the 2 letter word AD
I've seen this crossword clue in The Times
A
D
More clues leading to the result AD
- A German tabloid's taken over with no preparation
- Freely commercial library
- Groups influenced by media hoarded after promotion of last shop’s opening
- Left one in big band somehow playing without notes
- Impromptu remark from awkward lad I bowled
- A force on European border for the present
- One hint of departure poll to be considered further
- Appointed to represent unfortunate lad over article
- German newspaper article drawn up without preparation
- A bunch of well-read librarians wing it
Buddy explains!

The straight part
We know that "without preparation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case without preparation = ad.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 AD mean?
AD noun- Advertisement.
- Advantage
- (debating) advantage
- To, toward