Silly prank (5)
I believe the answer is:
antic
'silly prank' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for antic that I've seen before include "Mad act done for fun" , "Caper, trick" , "Frolic" , "Caper; grotesque (arch.)" , "Eccentric once" .)