Marks it "Collectors' items" (6)
I believe the answer is:
stamps
'collectors' items' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for stamps that I've seen before include "Philatelists collect them" , "Imprints" , "Brings one's foot down heavily on the envelope" , "Brings foot heavily to the ground, as in anger" , "Philatelists study them" .)