Invent what uranium and phosphorus do in combination (4,2)
I believe the answer is:
make up
'invent' is the definition.
('make up' can be a synonym of 'invent')
I can't explain the remainder of the clue.
(Other definitions for make up that I've seen before include "Prepare" , "Bury the hatchet" , "war paint?" , "Invent" , "become friends again" .)