Radioactive element, hot, decaying - bound to involve uranium (7)
I believe the answer is:
thorium
'radioactive element' is the definition.
(I've seen this before)
'hot decaying bound to involve uranium' is the wordplay.
'decaying' indicates anagramming the letters (the original order breaks down).
'bound' becomes 'rim' (both can mean a limit or boundary).
'to involve' means one lot of letters goes inside another (inserted letters get involved).
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'rim' going around 'u' is 'rium'.
'hot' anagrammed gives 'tho'.
'tho'+'rium'='THORIUM'
(Other definitions for thorium that I've seen before include "Radioactive metal, Th" , "chemical" , "Radioactive element -- rout him (anag)" , "source of radioactivity" , "Radioactive element, Th" .)