Start replacing second half of trading area like Slough (6)
I believe the answer is:
marshy
'like slough' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for marshy that I've seen before include "like Slough" , "Boggy, swampy" , "Like fenlands" , "Waterlogged ground" , "(Of ground) wet" .)