Jumper seen in sort of market? (4)
I believe the answer is:
flea
This clue is a double definition.
'jumper' is the first definition.
(something that jumps)
'sort of market?' is the second definition.
'seen in' acts as a link.
(Other definitions for flea that I've seen before include "Little jumper?" , "Insect (which you don't want in your ear!)" , "Leaping insects" , "irritant type" , "Small blood-sucking insect" .)