The most common and most likely answer to this crossword clue is the 4 plus 6 letter word HIGH SEASON
I've seen this crossword clue in Financial Times
H
I
G
H
S
E
A
S
O
N
More clues leading to the result HIGH SEASON
- Pongy child under the briny when most tourists come
- School’s dealt with each boy in a busy period
- Cheerful by water joining relation when it’s busiest
- When one sees the most tourists spring up, possibly?
- Sharp temper in peak tourist period
- Hotel? One’s a sight almost getting trashed in peak holiday period
Buddy explains!
The straight part
We know that "when it’s busiest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when it’s busiest = high season.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.