Sort of market for jumper (4)
I believe the answer is:
flea
I believe this is a double definition.
'sort of market' is the first definition.
'jumper' is the second definition.
(something that jumps)
'for' acts as a link.
(Other definitions for flea that I've seen before include "Jumping, blood-sucking insect" , "Circus insect?" , "Tiny circus performer?" , "beastie?" , "Leaping insect" .)