The most common and most likely answer to this crossword clue is the 6 letter word LINKED
I've seen this crossword clue in The Guardian
L
I
N
K
E
D
Buddy explains!

The straight part
We know that "joined up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joined up = linked.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 LINKED mean?
LINKED verb- To connect two or more things.
- (of a Web page) To contain a hyperlink to another page.
- To supply (somebody) with a hyperlink; to direct by means of a link.
- To post a hyperlink to.
- To demonstrate a correlation between two things.
- To combine objects generated by a compiler into a single executable.
- To skip or trip along smartly; to go quickly.
- Connected, either with links, or as if with links
- Having links (between modules, records etc)