The most common and most likely answer to this crossword clue is the 5 plus 10 letter word ROBIN GOODFELLOW
I've seen this crossword clue in The Sunday Telegraph
R
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B
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F
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More clues leading to the result ROBIN GOODFELLOW
- Lost in wood, goblin, elf or sprite
- Mischievous sprite contrived woe for old goblin
- Prankster in sport underneath sack
- Puck, below, is involved with indoor golf
- I am dressing 24, holding up party
- Take in lovely chap, mischievous character
- Bard’s character could be boring old fool, we fancy
- Puck's cross about game he follows
Buddy explains!
The straight part
We know that "mischievous character" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mischievous character = robin goodfellow.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.