Tiny margin — vibrissa (7)
I believe the answer is:
whisker
This clue is a double definition.
'tiny margin' is the first definition.
(I've seen this in another clue)
'vibrissa' is the second definition.
(synonyms)
(Other definitions for whisker that I've seen before include "narrow margin" , "Sensory hair of a cat" , "Bristle; small amount" , "Hair's breadth" , "Long stiff hair, of a cat say" .)