Tailor working in a marketplace (6)
I believe the answer is:
rialto
'marketplace' is the definition.
(I've seen this in another clue)
'tailor working' is the wordplay.
'working' is an anagram indicator.
'tailor' with letters rearranged gives 'RIALTO'.
'in a' acts as a link.
(Other definitions for rialto that I've seen before include "Area famous for 7 [ABRIDGE]" , "Dublin district, or Shakespeare's 'What news on the . . . . . .?'" , "District in Venice and in south Dublin" , "Bridge in Venice and Dublin" , "Venice area with famous bridge" .)