Jumper for which there may be a market (4)
I believe the answer is:
flea
This clue could be a double definition.
'jumper for which there' is the first definition.
'flea' can be an answer for 'jumper' (something that jumps). I'm unsure of the remainder of the definition.
'a market' is the second definition.
(flea market is a kind of market)
'may be' acts as a link.
Can you help me to learn more?
(Other definitions for flea that I've seen before include "Little jumping insect" , "irritant type" , "Wingless, jumping insect" , "Leaping insect" , "source of infestation" .)