The most common and most likely answer to this crossword clue is the 11 letter word GRASS-ROOTS
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Buddy explains!
The straight part
We know that "rank and file party supporters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rank and file party supporters = grass-roots.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 GRASS-ROOTS mean?
GRASS-ROOTS adjective- Of, or relating to people or society at the local level, particularly in politics, social movements, etc.; of the grass roots.