You'll need another one to make one (4)
I believe the answer is:
half
'one' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for half that I've seen before include "Measure of beer" , "Wife the better one (joc.)" , "50 per cent" , "Short distance - steal!" , "alcoholic drink" .)