Deuterium, say, is nothing, poet broadcast (7)
I believe the answer is:
isotope
'deuterium say' is the definition.
(deuterium is a kind of isotope)
'is nothing poet broadcast' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'broadcast' indicates anagramming the letters (letters scattered or cast around).
'poet' anagrammed gives 'tope'.
'is'+'o'+'tope'='ISOTOPE'
(Other definitions for isotope that I've seen before include "Oops, tie up the atom" , "Form of atomic nucleus" , "Atom with the same atomic number as another but with different numbers of neutrons" , "One of various forms of an elemental atom" , "One of the variants of an element sharing an atomic number but with differing chemical properties" .)