The most common and most likely answer to this crossword clue is the 8 letter word SPLICING
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More clues leading to the result SPLICING
Buddy explains!

The straight part
We know that "Joining" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joining = splicing.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 SPLICING mean?
SPLICING verb- To unite, as two ropes, or parts of a rope, by a particular manner of interweaving the strands, -- the union being between two ends, or between an end and the body of a rope.
- To unite, as spars, timbers, rails, etc., by lapping the two ends together, or by applying a piece which laps upon the two ends, and then binding, or in any way making fast.
- To unite in marriage.
- To unite as if splicing.
- To remove intron sequences from the pre-messenger RNA, and then join together exons.
- The act by which things are spliced.
- The point at which two things are spliced.