Atom of iron? That's not right! (3)
I believe the answer is:
ion
'atom' is the definition.
(ion is a kind of atom)
'iron? that's not right' is the wordplay.
'that's not' is a deletion indicator.
'right' becomes 'r' (common abbreviation).
'iron' with 'r' taken away is 'ION'.
'of' acts as a link.
(Other definitions for ion that I've seen before include "Electrically charged particle" , "Subatomic particle" , "Particle (phys.)" , "Charged atom/molecule" , "Electrically charged atom or group of atoms" .)