Cryptic crossword clue
One in spirit of party always backed line on the French Resistance 8

The most common and most likely answer to this crossword clue is the 8 letter word REVELLER

I've seen this crossword clue in Financial Times
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Buddy explains!

The straight part
We know that "One in spirit of party" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in spirit of party = reveller.
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 REVELLER mean?

REVELLER noun
  1. One who attends revels; a partygoer.

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