The most common and most likely answer to this crossword clue is the 11 letter word SUPERSCRIBE
I've seen this crossword clue in The Times
S
U
P
E
R
S
C
R
I
B
E
Buddy explains!
The straight part
We know that "put at the top" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case put at the top = superscribe.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 SUPERSCRIBE mean?
SUPERSCRIBE verb- To write on the exterior of, the surface of, or above.
- To write (something) on the exterior of an object, such as a document or an envelope.
- To address (an envelope etc.).