Pure uranium -- only deal is off! (9)
I believe the answer is:
unalloyed
'pure' is the definition.
('unalloyed' can be similar in meaning to 'pure')
'uranium only deal is off' is the wordplay.
'uranium' becomes 'u' (U is the chemical symbol for uranium).
'is off' indicates an anagram.
'only'+'deal'='onlydeal'
'onlydeal' is an anagram of 'nalloyed'.
'u'+'nalloyed'='UNALLOYED'
(Other definitions for unalloyed that I've seen before include "Not mixed with another metal" , "Pure, not a blend or mixture" .)