There's said to be a market for the thing (6)
I believe the answer is:
affair
'the thing' is the definition.
('affair' can be a synonym of 'thing')
'there's said to be a market' is the wordplay.
'there's' means one lot of letters go next to another.
'said to be' becomes 'af' (I can't explain this - if you can you should believe this answer much more).
'a market' becomes 'fair' (both can mean a place where things are sold).
'af'+'fair' is 'AFFAIR'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for affair that I've seen before include "Happening" , "Event; fling" , "Secret romance" , "Clandestine romance" , "Illicit liaison" .)