The most common and most likely answer to this crossword clue is the 14 letter word  LIGHT-HEARTED
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "Jolly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jolly = Â light-hearted.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  LIGHT-HEARTED mean?
 LIGHT-HEARTED adjective- Joyful, glad, taking pleasure in being alive; not depressed or sad.
- Enjoyably lacking of seriousness, not grave.