Two billion people (5)
I believe the answer is:
brace
'two' is the definition.
(brace is a kind of two)
'billion people' is the wordplay.
'billion' becomes 'b'.
'people' becomes 'race' (**).
'b'+'race'='BRACE'
(Other definitions for brace that I've seen before include "double" , "Carpenter's tool for a pair" , "Pair; hand drill" , "Pair of game-birds" , "Teeth straightener" .)