The most common and most likely answer to this crossword clue is the 4 plus 5 letter word LOVE CHILD
L
O
V
E
C
H
I
L
D
Buddy explains!
The straight part
We know that "issue comes before union" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case issue comes before union = love child.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.