Kumquat, for one (6)
I believe the answer is:
citrus
'kumquat for one' is the definition.
(kumquat is a kind of citrus)
This is all the clue.
(Other definitions for citrus that I've seen before include "Describing lemons, for example" , "Tree such as lemon or lime" , "Fruit tree genus" , "Eg, a lime tree" , "Eg, a lemon tree" .)