They're associated with heads and tails (3,4)
I believe the answer is:
top hats
'tails' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Perhaps you can see an association between them that I can't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for top hats that I've seen before include "Sort of covers" , "formal gear" .)