I can't arrange prank (5)
I believe the answer is:
antic
'prank' is the definition.
(I know that prank can be written as antic)
'i can't arrange' is the wordplay.
'arrange' indicates an anagram.
'i'+'cant'='icant'
'icant' anagrammed gives 'ANTIC'.
(Other definitions for antic that I've seen before include "Playful or mischievous act" , "old buffoon" , "Eccentric once" , "Caper; grotesque (arch.)" , "seen to be bizarre, at one time" .)