A little cough, merely marginal (3)
I believe the answer is:
hem
'a little' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hem that I've seen before include "It draws attention to" , "Bottom of a garment" , "Sewn edge to fabric" , "Stitched edge of garment" , "A stitched border of cloth" .)