Jumper found in this sort of market? (4)
I believe the answer is:
flea
I believe this clue is a double definition.
'jumper' is the first definition.
(something that jumps)
'this sort of market?' is the second definition.
'found in' is the link.
(Other definitions for flea that I've seen before include "Small parasitic insect" , "Little jumping insect" , "Bug" , "high-jumper" , "Leaping insects" .)