Small measure previously containing uranium (5)
I believe the answer is:
ounce
'small measure' is the definition.
(I've seen this in another clue)
'previously containing uranium' is the wordplay.
'previously' becomes 'once' (I've seen this before).
'containing' is an insertion indicator.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'once' going around 'u' is 'OUNCE'.
(Other definitions for ounce that I've seen before include "Wild cat or light weight" , "not weighing very much" , "one of several in a pound" , "Wildcat" , "Unit of weight - snow leopard" .)