A place where people gather in line (6)
I believe the answer is:
avenue
'line' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'a place where people gather' is the wordplay.
'place where people gather' becomes 'venue' (as in the venue for a meeting).
'a'+'venue'='AVENUE'
'in' is the link.
(Other definitions for avenue that I've seen before include "Road lined with trees" , "Tee-lined road" , "Broad tree-lined road" , "Road leading to a house" , "A broad thoroughfare as in New York" .)