The most common and most likely answer to this crossword clue is the 5 plus 5 letter word GREAT LAKES
I've seen this crossword clue in The Times
G
R
E
A
T
L
A
K
E
S
Buddy explains!
The straight part
We know that "Superior one joining up with four others" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case superior one joining up with four others = great lakes.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.