The most common and most likely answer to this crossword clue is the 5 letter word JUMBO
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More clues leading to the result JUMBO
Buddy explains!

The straight part
We know that "Very big" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very big = jumbo.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 JUMBO mean?
JUMBO noun- An especially large or powerful person, animal, or thing.
- A platform-mounted machine for drilling rock.
- Especially large or powerful.
- Short for mumbo jumbo.