9/14 across
When
filthier
traps
set?
(5,3,5)
When filthier traps set? (5,3,5)
hints
0
checks
0
hints
The whole clue is the definition (& lit or double definition).
How the clue works
The whole clue does double duty here — When filthier traps set? is both the definition (April Fools' Day being exactly when traps and pranks are set) and the wordplay, making it an &lit. "Set" is your anagram indicator, and sure enough, scrambling the letters of filthier traps gives you APRIL THE FIRST.