Small amount of uranium brought in long ago (5)
I believe the answer is:
ounce
'small amount' is the definition.
(I've seen this before)
'uranium brought in long ago' is the wordplay.
'uranium' becomes 'u' ('U' can be a synonym of 'uranium').
'brought in' indicates putting letters inside.
'long ago' becomes 'once' (I've seen this before).
'u' inserted within 'once' is 'OUNCE'.
'of' is the link.
(Other definitions for ounce that I've seen before include "small portion" , "Big cat of Asia" , "Unit of weight; cat" , "little weight" , "Small quantity - snow leopard" .)