Chief, playfully mischievous (4)
I believe the answer is:
arch
This clue is a double definition.
'chief' is the first definition.
('arch' can be a synonym of 'chief')
'playfully mischievous' is the second definition.
(Other definitions for arch that I've seen before include "This could be Wellington" , "Masonry structure for spanning an opening" , "Consciously or affectedly playful - under a bridge?" , "Self-consciously playful or teasing" , "architectural feature" .)