King wearing obscure jumper (4)
I believe the answer is:
frog
'jumper' is the definition.
(something which jumps)
'king wearing obscure' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'wearing' is an insertion indicator.
'obscure' becomes 'fog' (synonyms).
'r' placed into 'fog' is 'FROG'.
(Other definitions for frog that I've seen before include "Tailless hopper" , "Croaker?" , "in grass?" , "Amphibian - depression in brick" , "throat problem" .)