Playful, in the main (4)
I believe the answer is:
arch
I believe this clue is a double definition.
'playful' is the first definition.
(thesaurus)
'main' is the second definition.
(I've seen this before)
'in the' acts as a link.
(Other definitions for arch that I've seen before include "Marble, for example" , "Structure with a keystone" , "Affectedly playful in curved manner" , "Roguish - superior" , "Main" .)