What fool could be beaten by such a close margin (7)
I believe the answer is:
whisker
This clue is a double definition.
'what fool could be beaten by' is the first definition.
(fool is a dessert beaten with a whisk)
'such a close margin' is the second definition.
(as in beating by a whisker)
(Other definitions for whisker that I've seen before include "Long stiff hair, of a cat say" , "Sensory hair of a cat" , "Hair's breadth" , "Bristle; small amount" , "Long facial hair" .)