Cryptic crossword clue
Trees felled initially in part of S London, an annual tradition 5-7

The most common and most likely answer to this crossword clue is the 13 letter word FIRST-FOOTING

I've seen this crossword clue in Independent
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Buddy explains!

The straight part
We know that "an annual tradition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an annual tradition = first-footing.
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 FIRST-FOOTING mean?

FIRST-FOOTING noun
  1. The practice, during Hogmanay, of visiting friends and family, especially to be the first person to cross a home's threshold after midnight on New Year's Eve bearing a small gift.

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