The most common and most likely answer to this crossword clue is the 10 letter word CHAMPIONED
I've seen this crossword clue in Financial Times
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More clues leading to the result CHAMPIONED
Buddy explains!

The straight part
We know that "Was supportive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case was supportive = championed.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.
Crossword Answer definitions
CHAMPIONED verb- To promote, advocate, or act as a champion for (a cause, etc.).
- To challenge.