One opposing leader of crazy prank (5)
I believe the answer is:
antic
'prank' is the definition.
(I know that prank can be written as antic)
'one opposing leader of crazy' is the wordplay.
'one opposing' becomes 'anti' (prefix meaning 'opposing').
'leader of' indicates taking the first letters.
The first letter of 'crazy' is 'c'.
'anti'+'c'='ANTIC'
(Other definitions for antic that I've seen before include "Caper; grotesque (arch.)" , "Old clown" , "Absurd act" , "Ludicrous act done for fun" , "Playful caper" .)