The most common and most likely answer to this crossword clue is the 6 letter word COSIGN
C
O
S
I
G
N
Buddy explains!

The straight part
We know that "Jointly endorse" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jointly endorse = cosign.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 COSIGN mean?
COSIGN noun- The promotion of one musical artist (usually less successful) by another.
- To sign a document jointly with another person, sometimes as an endorsement.
- To agree with or endorse