A line, perhaps, in the sand where traders once went (9)
I believe the answer is:
samarkand
'where traders once went' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'a line perhaps in the sand' is the wordplay.
'line perhaps' becomes 'mark' (line is a kind of mark).
'in the' is an insertion indicator.
'a'+'mark'='amark'
'amark' placed within 'sand' is 'SAMARKAND'.
Can you help me to learn more?
(Other definitions for samarkand that I've seen before include "ancient Asian city" , "foreign city?" , "Silk Road city" , "where golden road ends?" , "Old city in Central Asia" .)