A line of footwear (8)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'a line' is the first definition.
(Plimsoll is an example)
'footwear' is the second definition.
(plimsoll is a kind of footwear)
'of' is the link.
(Other definitions for plimsoll that I've seen before include "Marine line", "Shipping legislator", "pump", "This line on ship's side measures degree of loading", "Canvas shoe", "Pump social reformer", "Sports shoe; a line", "'the sailors' friend'", "Light gym shoe and waterline" and "footwear?".)