Radioactive stuff? Putin men you ultimately suspect (9)
I believe the answer is:
neptunium
'radioactive stuff?' is the definition.
(radioactive element)
'putin men you ultimately suspect' is the wordplay.
'ultimately' says to take the final letters.
'suspect' indicates anagramming the letters (a suspicious or unusual spelling).
The final letter of 'you' is 'u'.
'putin'+'men'+'u'='putinmenu'
'putinmenu' is an anagram of 'NEPTUNIUM'.
(Other definitions for neptunium that I've seen before include "Element named after planet" , "Radioactive metal" , "scarce chemical" .)