The most common and most likely answer to this crossword clue is the 8 letter word SOLDERED
I've seen this crossword clue in The Daily Telegraph
S
O
L
D
E
R
E
D
More clues leading to the result SOLDERED
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 = soldered.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 SOLDERED mean?
SOLDERED verb- To join items together, or to coat them with solder
- To join things as if with solder.
- Fastened by means of solder.