Flood market primarily in exchange (5)
I believe the answer is:
swamp
'flood' is the definition.
(I know that swamp is a more specific form of flood)
'market primarily in exchange' is the wordplay.
'primarily' says to take the initial letters.
'in' means one lot of letters goes inside another.
'exchange' becomes 'swap' (swapping is a kind of exchanging).
The initial letter of 'market' is 'm'.
'm' going within 'swap' is 'SWAMP'.
(Other definitions for swamp that I've seen before include "Marshy land" , "Seasonally flooded low land" , "Bog, marsh" , "Seasonally-flooded lowland" , "Engulf" .)