Small amount of uranium to be imported by when? (5)
I believe the answer is:
ounce
'small amount' is the definition.
(an ounce is a small amount)
'uranium to be imported by when?' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'to be imported by' is an insertion indicator.
'when?' becomes 'once' ('once' can be a synonym of 'when').
'u' going inside 'once' is 'OUNCE'.
'of' acts as a link.
(Other definitions for ounce that I've seen before include "one of a number in the pound" , "Small amount (of old pound)" , "prowler" , "weight? Surely not" , "Small quantity - snow leopard" .)