Shipping line (8)
I believe the answer is:
plimsoll
This clue might be a double definition.
'shipping' is the first definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'line' is the second definition.
(I know that Plimsoll is a type of waterline)
Can you help me to learn more?
(Other definitions for plimsoll that I've seen before include "Mark on ship's side to indicate limit of loading" , "Pump social reformer" , "Shipping legislator" , "footwear?" , "Sports shoe; a line" .)