The most common and most likely answer to this crossword clue is the 10 letter word OFF-SEASON
I've seen this crossword clue in The Guardian
O
F
F
-
S
E
A
S
O
N
More clues leading to the result OFF-SEASON
Buddy explains!
The straight part
We know that "when trade is slack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when trade is slack = off-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.
What does OFF-SEASON mean?
OFF-SEASON noun- That part of the year when business is most slack and there are fewest clients or visitors
- Period of time when regular competitions are not being held, or when an athlete is not participating in competitions.