Uranium, now stolen, sadly restricts researcher’s latest contribution to fission (4,7)

'contribution to fission' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'uranium now stolen sadly restricts researcher's latest' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'sadly' indicates anagramming the letters.
'restricts' means one lot of letters goes inside another.
'latest' says to take the final letters.
The last letter of 'researcher' is 'r'.
'u'+'now'+'stolen'='unowstolen'
'unowstolen' is an anagram of 'slowneuton'.
'slowneuton' going around 'r' is 'SLOW NEUTRON'.

Can you help me to learn more?