The most common and most likely answer to this crossword clue is the 7 letter word FUSIBLE
I've seen this crossword clue in The Guardian
F
U
S
I
B
L
E
Buddy explains!
The straight part
We know that "amenable to union" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case amenable to union = fusible.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 FUSIBLE mean?
FUSIBLE noun- Any substance that can be fused or melted.
- Able to be fused or melted.