In contact with uranium, power working (4)
I believe the answer is:
upon
'in contact with' is the definition.
(I've seen this before)
'uranium power working' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'power' becomes 'p' (symbol in physics).
'working' becomes 'on' (on can mean working or activated).
'u'+'p'+'on'='UPON'
(Other definitions for upon that I've seen before include "Supported by (a surface)" , "voucher, business stopped" , "At a prescribed point in time" , "'''It came ... the midnight clear'' (carol) (4)'" , "Carried - on top" .)